window/wayland: Always update monitor for non-user ops

meta_window_wayland_update_main_monitor() would skip the monitor update
if the difference in scale between the old and the new monitor would
cause another monitor change.

While this is suitable when the monitor change results from a user
interactively moving the surface between monitors of different scales,
this can leave dangling pointers to freed monitors when this is
triggered by a change of monitor configuration.

Make sure we update the monitor unconditionally if not from a user
operation.

Closes: https://gitlab.gnome.org/GNOME/mutter/issues/189

src/wayland/meta-window-wayland.c

@@ -413,6 +413,12 @@ meta_window_wayland_update_main_monitor (MetaWindow *window,
       return;
     }
 
+  if (!user_op)
+    {
+      window->monitor = to;
+      return;
+    }
+
   from_scale = meta_logical_monitor_get_scale (from);
   to_scale = meta_logical_monitor_get_scale (to);