Blender Git Loki

USD Export: Armature export improvements.

Now including the root prim in the skinned mesh
skeleton relationship path. Also, added logic to
avoid nesting SkelRoot prims in the USD, as such
nesting causes skeleton binding to fail as well
as crashes in Create.

Now iterating over the deform groups of the
evaluated mesh when setting joint weights
and indices, to ensure the vertex group
data is valid.