glTF collision shapes and constraints extension

[JoshKlint]

I am working on two glTF extension definitions. The first will add support for collision shapes stored in the model file. This will allow the artist to define collision shapes in their 3D modeling application and then export it with the collision shape retained.

The second extension will add support for basic constraints (hinge, slider, etc.). This is mostly to make ragdoll setups easier.

The specification for the first one is here:
https://github.com/Leadwerks/glTFExtens ... /README.md

If you have any suggestions please let me know. Thank you.