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/backends/meta-monitor-manager.c

@@ -56,7 +56,6 @@
 #include "backends/meta-virtual-monitor.h"
 #include "backends/x11/meta-monitor-manager-xrandr.h"
 #include "clutter/clutter.h"
-#include "core/main-private.h"
 #include "core/util-private.h"
 #include "meta/main.h"
 #include "meta/meta-x11-errors.h"

src/backends/meta-viewport-info.c

@@ -23,7 +23,6 @@
 #include "config.h"
 
 #include "backends/meta-viewport-info.h"
-#include "core/main-private.h"
 #include "core/boxes-private.h"
 
 typedef struct _ViewInfo ViewInfo;