Scene graph - extract models and just use identifiers

[dwerner]

Going with a scene graph that uses Identity in place of actual meshes, Renderers could look up assets loaded by a referenced Identity. This could ease coupling of the renderer to the scene graph's loaded state.

Renderers should still be responsible for caching buffers, etc, internally, but perhaps now that we have multiple rendering modules, we should look at lifecycle management for renderers: State::on_render_load and State::on_render_unload should be specific to each renderer.