screen: Fetch the COW window up-front

XGetCompositeOverlayWindow is a round trip to the X server. We shouldn't
be doing it on every MapWindow.

src/core/screen-private.h

@@ -119,6 +119,8 @@ struct _MetaScreen
    * and restack them below a guard window. When using a compositor
    * this allows us to provide live previews of unmapped windows */
   Window guard_window;
+
+  Window composite_overlay_window;
 };
 
 struct _MetaScreenClass

src/core/screen.c

@@ -45,6 +45,7 @@
 #include "meta-cursor-tracker-private.h"
 
 #include <X11/extensions/Xinerama.h>
+#include <X11/extensions/Xcomposite.h>
 
 #include <X11/Xatom.h>
 #include <locale.h>
@@ -700,6 +701,8 @@ meta_screen_new (MetaDisplay *display,
   screen->starting_corner = META_SCREEN_TOPLEFT;
   screen->guard_window = None;
 
+  screen->composite_overlay_window = XCompositeGetOverlayWindow (xdisplay, xroot);
+
   reload_monitor_infos (screen);
 
   meta_screen_set_cursor (screen, META_CURSOR_DEFAULT);