window-actor: Fix rectangle coordinates in culling

The cull methods expect regions with 0,0 in the actor top-left corner,
whereas meta_window_get_frame_rect() returns a rectangle in workarea
coordinates.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/707

src/compositor/meta-window-actor.c

@@ -1388,6 +1388,7 @@ meta_window_actor_cull_out (MetaCullable   *cullable,
         {
           cairo_rectangle_int_t rect;
           meta_window_get_frame_rect (priv->window, &rect);
+          rect.x = rect.y = 0;
           cairo_region_subtract_rectangle (unobscured_region, &rect);
         }
     }