AVFoundation: Call stop() on VideoSurface when changing sources

Before we were reusing the active surface when we switched videos.  This
leads to an inconsitency in states, and the sourceRect property of the
video surface does not get updated.

Task-number: QTBUG-28655
Change-Id: Ie29bf1d9b1c11b6f51a869253c730202001c07cf
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>

src/plugins/avfoundation/mediaplayer/avfvideorenderercontrol.mm

@@ -182,6 +182,11 @@ void AVFVideoRendererControl::setLayer(void *playerLayer)
 
     m_playerLayer = playerLayer;
 
+    //If there is an active surface, make sure it has been stopped so that
+    //we can update it's state with the new content.
+    if (m_surface && m_surface->isActive())
+        m_surface->stop();
+
     //If there is no layer to render, stop scheduling updates
     if (m_playerLayer == 0) {
         m_displayLink->stop();