Copy Transforms: implement Invert, Fix Shear and more Mix options.

This constraint can be naturally viewed as a prototype for a future
4x4 matrix math node (or subset thereof), since its basic semantics
already is matrix assignment. Thus it makes sense to add math options
to this constraint to increase flexibility in the meantime.

This patch adds support for several operations that would be useful:

- An option to fix shear in the incoming target matrix.
- An option to invert the target matrix.
- More ways to combine target and owner matrix.

Shear is known to cause issues for various mathematical operations,
so an option to remove it at key points is useful. In the future node
system this would be a separate operation, but due to the limits of
the constraint stack it has to be built in for now.

Inverting a matrix is also an operation that can be useful to have.
For some uses it may be useful to invert components separately, so
implement this by checking the Mix mode setting to avoid UI options.

Finally, add two more ways to combine the matrices (multiplied by
two due to the necessity for the Before/After choice). Now there
are three combine modes:

Full implements regular matrix multiplication as the most basic option.

Split Channels combines location, rotation and scale separately.
Looking at D7547 there is demand for such a mode in some cases,
and even with nodes it's cumbersome to rig manually every time.

Finally, Aligned emulates the 'anti-shear' Aligned Inherit Scale mode,
and basically uses Full for location, and Split for rotation/scale.

Differential Revision: https://developer.blender.org/D9469

Armature: add B-Bone lengthwise scaling and custom handle scaling options.

In addition to the base bone transformation itself, B-Bones have
controls that affect transformation of its segments. For rotation
the features are quite complete, allowing to both reorient the
Bezier handles via properties, and to control them using custom
handle bones. However for scaling there are two deficiencies.

First, there are only X and Y scale factors (actually X and Z,
but this is the legacy naming), while lengthwise all segments
have the same scaling. The ease option merely affects the shape
of the curve, and does not cause actual scaling.

Second, scaling can only be controlled via properties, thus
requiring up to 6 drivers per joint between B-Bones to transfer
scaling factors from the handle bone. This is very inefficient.

This patch addresses these deficiencies by adding Length scale
inputs, and providing toggles to apply custom handle local scale
channels to the now four scale-related properties. The 'Length'
name is used to avoid confusion due to the X/Y vs X/Z naming.

The two Length scale inputs control the ratio between the lengths
of the start and end segments of the bone: although for convenience
two inputs are provided, the whole chain is still uniformly scaled
to fit the curve.

A Scale Easing option is provided to multiply the easing value
by the Length scale factors to synchronize them - this produces
a natural scaling effect where both the shape of the curve and
the scale is affected.

The second issue is addressed by providing toggles for each handle
that multiply each of the X, Z, Length and Ease values by the matching
Local Scale channel of the handle bone, thus replacing trivial drivers.
The Scale Easing option has no effect on this process since it's easy
to just enable both Length and Ease buttons.

Finally, this fixes a strange behavior where the segments were not
actually scaled in the Y direction to match their actual length, thus
producing gaps or overlap depending on the shape of the curve. For
transformation the change should be very small if enough segments
are used, but this will affect the results of the Copy Transforms
and Armature constraints, so a backwards compatibility option is
provided. Newly created bones default to the new behavior.

Differential Revision: https://developer.blender.org/D9870

Bone Overlay: support bone wireframe opacity settings.

When weight painting the bone overlay is extremely intrusive,
effectively requiring either extensive use of hiding individual
bones, or disabling the whole bone overlay between selections.

This addresses the issue by adding two bone opacity sliders that
are used for the 'wireframe' armature drawing mode. One directly
controls the opacity in a uniform way. The other one allows fade
based on the depth between the near and far clip planes in order
to provide an automatic visual cue about which bones are closest.

Constraints: refactor the D7437 patch adding Custom Space for constraints.

As mentioned in the comments to that patch, I had an idea for
a different way to do some technical aspects, but it was too
complicated to force changes in the original patch. Thus I submit
this follow up patch.

First, instead of modifying all the get_constraint_targets and
flush_constraint_targets callbacks, introduce wrapper functions
for accessing constraint targets, convert all code to use them,
and handle the new reference there uniformly for all constraints.

This incidentally revealed a place in the Collada exporter that
didn't clean up after retrieving the targets.

Also, tag the special target with a flag so other code can
handle it appropriately where necessary. This for instance
allows dependency graph to know that the Use B-Bone Shape
option doesn't affect this specific target.

Finally, rename and simplify the function for initializing the
custom space, and make sure it is called everywhere necessary.

Differential Revision: https://developer.blender.org/D9732

Constraints: add support for a new Owner Local Space for targets.

Add a new transformation space choice for bone constraints, which
represent the local transformation of the bone in the constraint
owner's local space.

The use case for this is transferring the local (i.e. excluding the
effect of parents) transformation of one bone to another one, while
ignoring the difference between their rest pose orientations.

Owner Local Space replaces the following setup:

* A `child` bone of the `target`, rotated the same as `owner` in rest pose.
* A `sibling` bone of the `target`, positioned same as `child` in rest
pose and using Copy Transforms in World Space from `child`.
* The `owner` bone constraint uses Local Space of `sibling`.

(This analogy applies provided both bones use Local Location)

Since the space list is getting long, this adds a couple of separators.

Differential Revision: https://developer.blender.org/D9493

Limit Rotation: add an Euler Order option and orthogonalize the matrix.

Since Limit Rotation is based on Euler decomposition, it should allow
specifying the order to use for the same reasons as Copy Rotation does,
namely, if the bone uses Quaternion rotation for its animation channels,
there is no way to choose the order for the constraint.

In addition, add a call to orthogonalize the matrix before processing
for the same reasons as D8915, and an early exit in case no limits are
enabled for a bit of extra efficiency.

Since the constraint goes through Euler decomposition, it would remove
shear even before the change, but the rotation won't make much sense.

Differential Revision: https://developer.blender.org/D9626

Force Fields: implement early filtering by the Affect flags.

Most fields have Affect Location and Rotation options that switch
off their effect, but they are only checked as the last step after
the force is already computed. It is more efficient to check it
when building the list of field objects, just like zero weight.
It is also possible to check the strength-related fields for 0.

As an aside, this adds Location to Texture fields (they don't
handle rotation) and both Location & Rotation checkboxes to
Fluid Flow. Boid and Curve Guide remain without options for
now as they are completely different from others.

Differential Revision: https://developer.blender.org/D10087

Depsgraph: connect up drivers on various physics properties.

It seems drivers for physics properties weren't being linked to
evaluation nodes. This connects settings used by modifiers
to Geometry; particle settings and rigid body data to Transform
which seems to contain rigid body evaluation; and force fields
to object Transform, since fields can exist on empties.

Differential Revision: https://developer.blender.org/D10088

Surface Deform: fix binding vertex artifacts causing spikes.

There are two issues here. First, like in T81988 there are cases
where the modifier would deform some vertices immediately after
bind. This is caused by wrong assumptions in the code about the
possible relative angles between various vectors, which can cause
negative weights that don't blend correctly to appear.

Specifically, it seems originally the code assumes that the
centroid-point vector in the polygon plane lies somewhere
between the mid-edge vectors. This is however not necessarily
the case for distant vertices, because the polygon is not
guaranteed to be truly planar, so normal projection may be
a bit off. The code has to use signed angles and checks to
support all possible angular arrangements.

The second issue is very thin and long triangles, which tend
to be very spatially unstable in their thin dimension, resulting
in excess deformation. The code was weighting distance using
the distances between the centroid and the mid-edge points, which
in this case end up as nearly opposite vectors of sizable length
and don't correctly represent how thin the triangle actually is.
It is thus better to use centroid-to-line distances, and an
additional even stricter value for the midpoint that will use
only 3 vertices at evaluation time.

Differential Revision: https://developer.blender.org/D10065

Merge branch 'blender-v2.92-release'