B-Bone shape is a non-trivial computation, so access to
the results would be useful for Python scripts working with
B-Bones, e.g. rig generation.
This exposes both final segment matrices, and the tangent
vectors computed from the custom handle bones.
Since the handle tangents use the axis+roll orientation math
of edit bones, add matrix conversion static methods to Bone.
Reviewers: campbellbarton
Differential Revision: https://developer.blender.org/D3983
a58f0eea4f