MetaSurfaceActorWayland: Unset the MetaWaylandSurface pointer when it goes away

We may access it during painting even if it has been freed. For now,
manually unset it during the MetaWaylandSurface cleanup; in the future
make MetaWaylandSurface a GObject and make the surface pointer a weak
reference.

https://bugzilla.gnome.org/show_bug.cgi?id=744453

src/wayland/meta-wayland-surface.c

@@ -802,6 +802,9 @@ wl_surface_destructor (struct wl_resource *resource)
   if (surface->input_region)
     cairo_region_destroy (surface->input_region);
 
+  meta_surface_actor_wayland_surface_destroyed (
+    META_SURFACE_ACTOR_WAYLAND (surface->surface_actor));
+
   g_object_unref (surface->surface_actor);
 
   meta_wayland_compositor_destroy_frame_callbacks (compositor, surface);