window/x11: Add flag to thaw commits after resize

To be able to thaw commits following a resize that might have frozen
commits, to keep freezes and thaws even, we need a way to tell whether
a repaint should also thaw commits.

Add a flag to `MetaWindowX11` and the appropriate functions to set and
query it.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/942

src/x11/window-x11-private.h

@@ -75,6 +75,9 @@ struct _MetaWindowX11Private
   MetaIconCache icon_cache;
   Pixmap wm_hints_pixmap;
   Pixmap wm_hints_mask;
+
+  /* Freeze/thaw on resize (for Xwayland) */
+  gboolean thaw_after_paint;
 };
 
 G_END_DECLS