Add 'instance' transform

[donmccurdy]

Given a model in which a particular mesh is repeated many times in the scene, with different transforms, output a model using EXT_gpu_mesh_compression to instance the mesh. Ideally with options for tiling the instances for better culling. Some ideas for sample models:

• https://sketchfab.com/search?features=downloadable&q=grass+field&sort_by=-relevance&type=models
• https://sketchfab.com/3d-models/pine-forest-ece69535f7584e099488f65f2072264e
• https://sketchfab.com/Somersby

[donmccurdy]

Steps:

1. Gather groups of meshes that can be instanced, assuming a dedup pass has already been done.
2. Split each group into cells, if that option is enabled.
3. For each instance compute the transform
4. Write batches ... to scene root? Use common parent for cells from the same group? To same parent as first instance in the batch?