gltfpack: Implement saving to .gltf/.glb
gltfpack: Implement saving to .gltf/.glb

We currently assume that we're only going to use one buffer, and save
the same contents to .gltf/.glb as requested. When saving to .gltf, the
buffer is saved into <name>.bin.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer