multiple uv map export support

[Aeva]

Currently, the blender exporter only allows for one uv map to be present.

What would be nice instead, is if a collection of objects could have their own local UV maps, but then also there be a lightmap unwrap for the entire scene, using the texture atlas functionality.

This could allow for some interesting compositing, like projecting decals onto objects while creating a level, as it could also be used to do high quality light map bakes.

This might require some revisions to the jta importer.