Set default values for optional vertex attributes so all objects can use all shaders.
Load default texture if url is nil.

This can happen if no default texture image is specified. The in-code default is typically a worse idea, but at least it exists.
Users can change shaders (render groups).

This fixes #33. And it's amazing what one can do with this. Export of
the new models sounds like a good idea. Anyway, lots of new code, but
the general principles shouldn't be too surprising.