Support morphed TEXCOORD_n and COLOR_n attributes

[schell]

The spec mentions that there can be n TEXCOORD and/or COLOR morph target buffers. gltf should support this. I can contribute this at some point, but I wanted to create this ticket to track that feature in case someone beats me to it :)

[chubei-urus]

Hi @schell are you working on this? My organization also needs this. If you haven't started yet I'll go ahead and implement it.

[schell]

I have not started yet @chubei-urus. Feel free to take it :)

[chubei-urus]

I put together a quick hack in urusgraphics@57c41a2, but I don't think it's ever going to be merged because it breaks compatibility.

[schell]

Ah, ok, I'll put this back on my todo list, then :)