SkeletonDefinition Implementation

[marstaik]

Allows Skeletons to share a common definition, as well as export skeletons for future use in other scenes.

[marstaik]

Quick preview about what this SkeletonDefinition allows users to do:
https://f.staik.net/f/2cx.mp4

[fire]

Imagine you want to export clothes when you don't have all the bones.

A master skeleton scene has every bone. So this skeleton structure should also have all the information to look up the name of the bones in the master skeleton.

The skin will do the remapping of the skeleton. The bone indices are indices to the skin which remap to indices in the skeleton.

Assumption: The new scene's skeleton has existing mesh instances-likes.

I propose that SkeletonDefinition will follow every assigned mesh and create such skins after the import of the SkeletonDefinition.

[fire]

I've updated the pull request for the crash fix and to implement the above feature.

fire@2495689

Visually, it's importing skeleton defintions.

[marstaik]

Replaced partially by #36409

You can also now use scene instances of a skeleton to achieve the same effect, as meshes wont fail to bind by bone name anymore.