Add support for 32-bit indices

[zellski]

The tool still invariably creates 16-bit indices, and thus also often splits large FBX meshes into smaller glTF meshes. I'm currently thinking:

• FBX2glTF --use-short-indices
  • force 16-bit indices
• FBX2glTF --use-long-indices # force 32-bit indices
  • force 32-bit indices

Without either option, the tool should choose 16-bit or 32-bit on a mesh-by-mesh basis, depending on which would be most economical.