protocol: set_desync should flush

wl_subsurface.set_desync should apply the cached wl_surface state.
Otherwise, the sub-surface may be stuck: a commit on the parent surface,
if desynchronized, will not commit the sub-surface because it is
desynchronized, too. A commit on the sub-surface may not happen, if it
is waiting for the frame callback from the previous commit.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

protocol/subsurface.xml

@@ -229,6 +229,9 @@
 	Note: even if a sub-surface is set to desynchronized, a parent
 	sub-surface may override it to behave as synchronized. For details,
 	see wl_subsurface.
+
+	If a surface's parent surface behaves as desynchronized, then
+	the cached state is applied on set_desync.
       </description>
     </request>