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/tests/meta-monitor-test-utils.c

@@ -216,7 +216,7 @@ logical_monitor_from_layout (MetaMonitorManager *monitor_manager,
     {
       MetaLogicalMonitor *logical_monitor = l->data;
 
-      if (meta_rectangle_equal (layout, &logical_monitor->rect))
+      if (mtk_rectangle_equal (layout, &logical_monitor->rect))
         return logical_monitor;
     }