Temporary Hack: provide B-Bone scale versioning for files with old patch.

Run the versioning code for the conversion of bbone scale to an xyz
vector if it has fields that correspond to the old version of the
patch before that change requiring versioning.

The actual Y (length) scale value from the old patch isn't versioned
and will be lost, requiring manual fixing.

Animation: support filtering for curves with a cycle period mismatch.

Since the looping behavior is defined per curve rather than at
action level, it is possible for curve loop periods to get out of
sync with each other. This commit adds an option to compare curves
against the frame range specified in the action, and treat those
with a mismatch as errors for the purpose of F-Curve filtering.

When enabled, the check verifies that curves within the action
have valid cyclic extrapolation, and the action period evenly
divides by the curve period (since a curve looping at e.g. half
of the action period length still repeats in sync with the action).

Animation: allow specifying a custom frame range for actions.

Some operations, e.g. adding a new action strip to NLA, require
knowing the active frame range of an action, but currently it can
only be deduced by scanning the keyframes of the curves within it.
This is not ideal if e.g. curves are staggered for overlap.

As suggested by Nathan Vegdahl, this patch adds Action properties
that allow manually specifying its active frame range, and whether
it is intended to be cyclic. The settings are exposed in a popover
in Action Editor. When enabled, the range is highlighted in the
background using a striped fill to distinguish it from the solid
filled regular playback range.

When set, the frame range is used by NLA. In addition, the Cycle-Aware
Keying option automatically sets up F-Curves newly added to a cyclic
action to use cyclic extrapolation with the correct period.

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.

Copy Transforms: implement Remove Target 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 remove shear in the incoming target matrix.

Shear is known to cause issues for various mathematical operations,
so an option to remove it at key points is useful.

Constraints based on Euler like Copy Rotation and Limit Rotation
already have always enabled shear removal built in, because their
math doesn't work correctly with shear.

In the future node system shear removal would be a separate node
(and currently Limit Rotation can be used as a Remove Shear constraint).
However removing shear from the result of the target space conversion
before mixing (similar to Copy Rotation) has to be built into
Copy Transforms itself as an option.

- More ways to combine the target and owner matrices.

Similar to multiple Inherit Scale modes for parenting, there are
multiple ways one may want to combine matrices based on context.
This implements 3 variants for each of the Before/After modes
(one of them already existing).

- Full implements regular matrix multiplication as the most basic
option. The downside is the risk of creating shear.
- Aligned emulates the 'anti-shear' Aligned Inherit Scale mode,
and basically uses Full for location, and Split for rotation/scale.
(This choice already existed.)
- Split Channels combines location, rotation and scale separately.

Looking at D7547 there is demand for Split Channels in some cases,
so I think it makes sense to include it in Copy Transforms too, so that
the Mix menu items can be identical for it and the Action constraint.

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

Armature: apply Y scale to B-Bone segments.

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.

Constraints: support a new Local Space (Owner Orientation) for targets.

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

The use case for this is transferring the local (i.e. excluding the
effect of parents) motion 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, also add a couple of separators.

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

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

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