Simplify VAO setup code

Previously buffer binding semantics caused complications, as index buffer
was unbound while a mesh was bound so batches couldn't be updated.  Now
that buffers behave better with bounds meshes, batches can take care of
themselves again.

Triggering the setup from bind() instead of draw() gets rid of another
bunch of complications, since it will only be called when the VAO is going
to be actually used.