wayland: Sync surface actor state when changing main monitor

In order to switch to the correct surface actor scale given the monitor
the surface is on, without relying on the client committing a new state
given some other side effect, sync the surface actor state when the main
monitor associated with the corresponding window changed.

https://bugzilla.gnome.org/show_bug.cgi?id=744933

src/core/window-private.h

@@ -482,6 +482,8 @@ struct _MetaWindowClass
   gboolean (*update_icon)        (MetaWindow       *window,
                                   cairo_surface_t **icon,
                                   cairo_surface_t **mini_icon);
+  void (*main_monitor_changed)   (MetaWindow *window,
+                                  const MetaMonitorInfo *old);
 };
 
 /* These differ from window->has_foo_func in that they consider