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/workspace.c

@@ -1032,7 +1032,7 @@ strut_lists_equal (GSList *l,
       MetaStrut *b = m->data;
 
       if (a->side != b->side ||
-          !meta_rectangle_equal (&a->rect, &b->rect))
+          !mtk_rectangle_equal (&a->rect, &b->rect))
         return FALSE;
     }