Allow GLTF export to divide objects so each can be a single color for use in Enduvo

[Tom Goddard]

The Enduvo VR presentation software does not appear to handle texture or vertex colors in GLTF files, only single color objects. We could divide ribbons into separate GLTF objects for each residue so different residues can have different colors, likewise for atoms, to try to provide color GLTF models for use in Enduvo.

Probably before attempting this it makes sense to try other scene file formats that Enduvo accepts to see if ChimeraX can provide a different format (e.g. OBJ) that would preserve colors.