Also, split it into its own file - which was the original reason for
looking at this code, the rewrite was an unintentional side effect.

This changes the context to create surfaces on demand.
So whenever the compositor holds onto a surface while GDK wants to
render, it just creates a new surface. If the compositor releases
surfaces, we will retain one for the next frame to be rendered, but free
all extra ones.
This way, we should get to a stage where we have exactly as many
surfaces as needed and never allocate/free any.