Remaining issues with the gltf loader

I've tried all the gltf samples models from [here](https://github.com/KhronosGroup/glTF-Sample-Models) and while many already load, here's the list of remaining issues:

## TODO:

- ~Animations~
- Coating (ClearCoatTest)
- Environment (EnvironmentTest)
- Morph primitives (MorphPrimitivesTest)
- Multiple UVs (MultiUVTest)
- Material Variants (SheenChair, SheenCloth)
- Transmission (TransmissionTest)
- ~Two Sided (TwoSidedPlane)~
- Texture Transform (TextureTransformMultiTest, TextureTransformTest)
- Loading lights
- KHR_draco_mesh_compression
- glTF-Quantized
- ~"Embedded" gltf variants (inline data URIs)~
- ~Cameras~
- ~Vertex colors (VertexColorTest)~
- ~data URIs in images~
- ~Missing Vertex_Normal~
- ~Missing UV~

## Bugs:
- ~glTF-Sample-Models/2.0/SheenCloth/glTF/SheenCloth.gltf (range end index 263168 out of range for slice of length 262144)~
- glTF-Sample-Models/2.0/SimpleSparseAccessor/glTF/SimpleSparseAccessor.gltf (subtract with overflow)
- ~glTF-Sample-Models/2.0/Box With Spaces/glTF/Box With Spaces.gltf (failed to load path)~