Add a warning for recursive emit_event

When event delivery is invoked by synthetic events through
clutter_do_event from inside an event handler clutter was silently
ignoring it, this warning will hopefully help resolving some issues.

clutter/clutter-main.c

@@ -1748,7 +1748,10 @@ emit_event (ClutterEvent *event,
 
   /* reentrancy check */
   if (lock != FALSE)
+    {
+      g_warning ("Tried emitting event during event delivery, bailing out.n");
       return;
+    }
 
   lock = TRUE;