scene: Fix SurfaceItemWayland::freeze()
When a wl_surface is unmapped, we need to stop updating the buffer in SurfacePixmapWayland.
However, SurfaceItemWayland::freeze() doesn't unset m_surface, so the SurfacePixmapWayland keeps updating the buffer even after the surface is unmapped. This results in some closed windows losing their contents when playing a window closing animation.