Added 'handled' return state to window update handler.

tests/testapp.py

@@ -127,6 +127,7 @@ def create_context_window(w, h, callback):
         """Synchronize overlay with GTK window."""
         overlay.set_size(*window.get_size())
         refresh()
+        return True
 
     def handle_key(win, event):
         """Handle key event. 'f' toggles fullscreen, 'b' toggles bounding