Optimize VAO/VBO usage for minimal state changes.

This is similar to what we had earlier to just reuse the VAO,
but now with correct bindings no matter what vertex attributes
are assigned to what index, so that the (new) test passes.

This is actually slightly more efficient than what we had before,
since we don't look up the attributes by text anymore, and don't
reupload the VBO for each frame anymore. In practice, the effects
should be small.