glTF support

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](https://sketchfab.com/models?features=downloadable+staffpicked&sort_by=-likeCount&type=models).