Implement a generic "size change" mechanism

A much less hacky version of maximize / unmaximize is reimplemented
in terms of this, but it could also eventually be used for fullscreen /
unfullscreen, and tile / untile.

src/core/window-private.h

@@ -78,6 +78,7 @@ typedef enum
   META_MOVE_RESIZE_RESIZE_ACTION     = 1 << 3,
   META_MOVE_RESIZE_WAYLAND_RESIZE    = 1 << 4,
   META_MOVE_RESIZE_STATE_CHANGED     = 1 << 5,
+  META_MOVE_RESIZE_DONT_SYNC_COMPOSITOR = 1 << 6,
 } MetaMoveResizeFlags;
 
 typedef enum