Support glTF Skinning for Skeletal Animation

[sunjay]

Update (Jan 18, 2020): Animation work is tracked in #18. This issue is now tracking the work on supporting glTF skins. This is a big part of skeletal animation and will hopefully allow us to have a fully implemented animation engine by the time #18 is done.

Skins: https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#skins

[JLSJamesShi]

Keep this github issue in mind for edge cases:
KhronosGroup/glTF#1275

[sunjay]

This is going to be @JLSJamesShi's main focus after the other animation issues he is assigned to have been completed. All of the animation issues are going to require a large amount of work, and this is the largest by far because of the theoretical knowledge required and the edge cases I am anticipating when we implement skinning.