File under #138795 showed several issues, which, while investigating
them, led to also fixing some other issues.
- FBX files can contain non-bone nodes in between actual bone nodes
("fake bones" as they used to be called in Python importer). Handling
this case was missing in the new importer.
- Due to above, some armatures had what appeared like multiple
"root bones" inside them, which led to crashes while importing
animations.
- Meshes with multiple armature modifiers (multiple skin deformers
in FBX) were not handled correctly, see
https://projects.blender.org/blender/blender-addons/issues/45171
for when the same issue was fixed in the Python importer.
Extended test coverage to encompass the above.
Pull Request: https://projects.blender.org/blender/blender/pulls/138992
de5d0cfdc5