join() should merge primitives within an instanced mesh

[donmccurdy]

As discussed in #1051, the join() operation bails out when it sees an instanced mesh (EXT_mesh_gpu_instancing), because it knows it won't be able to join that content to anything else in the scene. However, we could join primitives within an instanced mesh. The implementation of join() should be improved to do that where possible.

[kzhsw]

Should this be optional or not?

[donmccurdy]

The scenario I'm seeing is one where a single mesh, containing many primitives, is instanced in many places. The primitives within that mesh can be joined without affecting the benefits of instancing.

When the user has opted to run join(), I can't think of any reason they wouldn't also want this, though I'd be happy to discuss it if anyone needs alternatives.