Hide the video overlay when the QML element is hidden

An display invalid rect is used for invisible QML
items.

Change-Id: Ifb2a25f1c5387ab8cef1359ac6c3e2f90a42cd10
Reviewed-by: Dmytro Poplavskiy <dmytro.poplavskiy@gmail.com>

src/imports/multimedia/qdeclarativevideooutput_window_p.h

@@ -65,6 +65,7 @@ public:
 
 private:
     QPointer<QVideoWindowControl> m_videoWindowControl;
+    bool m_visible;
 };
 
 QT_END_NAMESPACE