playback/player: qt: Return a new reference to the renderer from VideoRenderer::renderer()

gst_player_new_full() takes ownership of it, and that's where it is usually
used. Without this we would create assertions on application shutdown.

Fixes #129

playback/player/qt/qgstplayer.cpp

@@ -567,7 +567,7 @@ void Player::setPosition(qint64 pos)
 
 GstPlayerVideoRenderer *VideoRenderer::renderer()
 {
-    return renderer_;
+    return static_cast<GstPlayerVideoRenderer*> (gst_object_ref (renderer_));
 }
 
 VideoRenderer::VideoRenderer()