Add GLTF support

[tychedelia]

We want to support loading GLTFs to interface with tools like Blender! This is going to be tricky. Bevy can load GLTFs but gives the user a Scene containing whatever objects happened to be in that file. In our terms, that means we'll get a jumble of Geometry, Material, Image, etc. We'll want to figure out how to expose those to users, probably by looking them up in a GLTF handle via a path.