wayland/xdg-popup: Always use monitor of toplevel

Always use the monitor of the toplevel surface's window, so that the
popup menu and the parent will always have the same scale. This fixes
the dimensions sent in the xdg_popup configure event.

https://bugzilla.gnome.org/show_bug.cgi?id=771841

src/wayland/meta-wayland-xdg-shell.c

@@ -1464,6 +1464,7 @@ xdg_surface_constructor_get_toplevel (struct wl_client   *client,
 
   window = meta_window_wayland_new (meta_get_display (), surface);
   meta_wayland_surface_set_window (surface, window);
+  meta_window_update_monitor (window, FALSE);
 }
 
 static void