scene: Freeze SurfaceItem tree when the Window is closed
If the wl_surface is unmapped, the compositor should unmap the window. Most clients don't do it, and instead destroy the wl_surface or the surface role object or both. A very tiny fraction of clients actually close the window by unmapping the wl_surface. Either way, it's worth handling that case because xdg-shell protocol says that the clients are allowed to do it.