Blender Git Loki

Armature: fix B-Bone deformation blending artifacts with Preserve Volume.

The double quaternion blending method in addition to the deformation
matrix of each bone requires their rest matrices. For ordinary bones
this literally should use the bone rest matrix without any ambiguity.

However, it was also using the bone rest matrix for all of its
B-Bone segments, which is incorrect and causes strange deformation
in some cases involving extreme non-uniform scale, especially
at boundaries between different B-Bones. There is also a similar
known issue that happens with scale at bending joints, and this
fix reduces the distortion when both bones are B-Bones.

This changes both the Armature modifier and the Armature constraint
to use the actual segment rest matrices. Unlike bones, these can have
scale even in rest pose, so normalization is required.

Differential Revision: https://developer.blender.org/D10003