mutter: Completely replace MetaRectangle with MtkRectangle

There are still various helpers that might be worth to move to mtk as
well

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

src/tests/monitor-store-unit-tests.c

@@ -53,7 +53,7 @@ typedef struct _MonitorStoreTestCaseMonitor
 
 typedef struct _MonitorStoreTestCaseLogicalMonitor
 {
-  MetaRectangle layout;
+  MtkRectangle layout;
   float scale;
   MetaMonitorTransform transform;
   gboolean is_primary;