update the cursor on next commit after xdg surface map

we can't update the cursor properly upon map, because windows may
not have committed a buffer by then.

deferring cursor updates until commit lets us find the buffer in the
scene tree and see if the pointer is within it.

include/view.h

@@ -46,6 +46,7 @@ struct view {
 
 	bool mapped;
 	bool been_mapped;
+	bool committed_since_mapped;
 	bool ssd_enabled;
 	bool minimized;
 	bool maximized;