glTF support

[hunterloftis]

pbr currently supports three 3D data formats:

1. Wavefront .obj files (meshes)
2. Wavefront .mtl files (materials)
3. Radiosity .hdr files (environment maps)

These are all old formats without physically-based materials in mind. A newer format, [glTF], is an easy-to-read format for physically-based models and scenes. This would open up many more resources for 3d assets.

[laikee99]

There's a golang gltf pkg: https://github.com/qmuntal/gltf
Is it possible to integrate with this package?