With this change the render backend only sees the state of the scene
that it can use for frame building and the scene builder thread manages
its own scene.
This will allow us to perform low priority scene building and rasterization
asynchronously without having to involve the render backend thread.

All documents currently always have the same config.

From now on, generate frame means having the frame builder generate a frame (but not necessarily render it), and render means asking the renderer to render a frame.

This is to avoid confusion with the previous meaning of render which was about building the frame rather than rendering it.