Allow overrides for cloth, collision and force field properties.

Allow overriding simple properties of cloth simulations, colliders
and force fields. Vertex group and shape key selectors in cloth are
still not overridable since they are tied to mesh data.

Force fields have a number of physical fields shared between multiple
RNA fields. Until they are decoupled, they will produce redundant
overrides, and cannot have different hard range limits.

Bone Overlay: support bone wireframe opacity depth fade.

Add an option that allows fade based on the depth from the camera,
using exponential decay with the slider specifying the 'half-life'
depth. This is intended as a way to automatically hide bones
in distant parts of the mesh while focused on a specific part.

Bone Overlay: support changing bone wireframe opacity.

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 a bone opacity slider that
is used for the 'wireframe' armature drawing mode. It directly
controls the uniform opacity as a straightforward option.

Differential Revision: https://developer.blender.org/D11804

RNA: add a property flag to display exact integer values without fraction.

Add a flag to display exact integer values of a floating point field
without a fraction, for use in case when the value can technically be
fractional, but most commonly is supposed to be integer.

The fraction is discarded in the normal display mode and when copying
the value to clipboard, but not when editing to remind the user that
the field allows fractions.

Also, fix a precision issue when stepping down from 1 to 0 via the
left decrement button and step 100 results in a small nonzero value.

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 that have cycle issues.

It is possible to have curves with cyclic extrapolation that
have a mismatch in their end keyframes, causing a jump.

Also, 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 manual frame range specified in the action, and treat
any mismatches as errors for the purpose of F-Curve filtering.

When enabled, the check verifies that end values of cyclic curves
match, curves within a cyclic 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).

Ref: D11803

Differential Revision: https://developer.blender.org/D13349

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

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.

Force Fields: implement new true power and custom falloff options.

The 'power' falloff option in Blender force fields does not actually
generate a true power falloff function, as pointed out in D2389.
However, that patch adds a special 'gravity' falloff option to Force
fields, without addressing the shortcoming in the common options.

The reason for not using the true curve in the options, as far as
one can tell, is that the power curve goes up to infinity as the
distance is reduced to 0, while the falloff options are designed
so that the maximum value of the curve is 1.

However, in reality forces with a power falloff don't actually go
to infinity, because real objects have a nonzero size, and the force
reaches its maximum at the surface of the object. This can be used
to integrate an option to use a true power falloff with the design
of falloff settings, if it requires a nonzero 'minimum' distance
to be set, and uses a curve that reaches 1 at that distance.

Since this is adding a new feature to the minimum distance value,
it is also a good opportunity to add a feature to the maximum
distance. Specifically, the new options can be used to apply
arbitrary brush-style falloff curves between min and max,
including a fully custom curve option. When used together with
power falloff, the two curves are multiplied together.

While the true power option allows creating more physically
correct forces, the custom curves aid artistic effects.

Differential Revision: https://developer.blender.org/D8075