Add gltf support for sparse accessors and non indexed meshes.

[maoliver-amd]

Adds support for sparse buffer accessors Fixes loading meshes that dont define an index buffer (it actually just defines an default index buffer now - this was actually easier than not creating an index buffer and makes it easier to add support for triangle strips/fans later) Fixes loading files that dont define a default scene.

[gboisse]

Thanks for the fixes.