Implement QGstreamerVideoRenderer::stopRenderer()

Surface has to be stopped when
QGstreamerVideoRendererInterface::stopRenderer() is called.
This fixes a media player issue when video output is not cleared
when media is set to 0.

Change-Id: Ifdc851797519dcdf95db86e8dcec93b3866c388e
Reviewed-by: Dmytro Poplavskiy <dmytro.poplavskiy@nokia.com>

src/multimedia/gsttools_headers/qgstreamervideorenderer_p.h

@@ -62,6 +62,7 @@ public:
 
     GstElement *videoSink();
 
+    void stopRenderer();
     bool isReady() const { return m_surface != 0; }
 
 signals: