main: Move rect related macro to util-private.h

No reason that it should be in main-private.h, lets place it in
util-private.h. This also mean we can remove main-private.h.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1833>

src/core/stack.c

@@ -32,7 +32,6 @@
 #include "backends/meta-logical-monitor.h"
 #include "cogl/cogl.h"
 #include "core/frame.h"
-#include "core/main-private.h"
 #include "core/meta-workspace-manager-private.h"
 #include "core/window-private.h"
 #include "meta/group.h"