mtk: Move Rectangle.equal from Meta

Moves equal the equal function and removes the clutter equivalent. The
tests were not moved until all the remaining helper functions are moved.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3128>

src/core/window.c

@@ -2813,7 +2813,7 @@ meta_window_is_monitor_sized (MetaWindow *window)
       meta_window_get_frame_rect (window, &window_rect);
       meta_display_get_monitor_geometry (window->display, window->monitor->number, &monitor_rect);
 
-      if (meta_rectangle_equal (&window_rect, &monitor_rect))
+      if (mtk_rectangle_equal (&window_rect, &monitor_rect))
         return TRUE;
     }