gltfpack: Implement support for 16-bit index buffers
gltfpack: Implement support for 16-bit index buffers

We now automatically write 16-bit or 32-bit index data as appropriate.

Due to index compression, 16-bit indices don't save space in compressed
blobs, but they save GPU memory and reduce the size of uncompressed
glb files.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer