VideoOutput: release video frames when the video surface is stopped.

It's necessary to release video frames during media pipeline shutdown
or reconfiguration.

Change-Id: I386ad4d173b8731f257ec9272ef8c46a27769bd0
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>

src/imports/multimedia/qdeclarativevideooutput.cpp

@@ -95,6 +95,12 @@ public:
         return QAbstractVideoSurface::start(format);
     }
 
+    void stop()
+    {
+        m_item->stop();
+        QAbstractVideoSurface::stop();
+    }
+
     virtual bool present(const QVideoFrame &frame)
     {
         if (!frame.isValid()) {
@@ -272,10 +278,18 @@ void QDeclarativeVideoOutput::_q_updateMediaObject()
 
 void QDeclarativeVideoOutput::present(const QVideoFrame &frame)
 {
+    m_frameMutex.lock();
     m_frame = frame;
+    m_frameMutex.unlock();
+
     update();
 }
 
+void QDeclarativeVideoOutput::stop()
+{
+    present(QVideoFrame());
+}
+
 /*!
     \qmlproperty enumeration VideoOutput::fillMode
 
@@ -353,6 +367,8 @@ QSGNode *QDeclarativeVideoOutput::updatePaintNode(QSGNode *oldNode, UpdatePaintN
 {
     QSGVideoNode *videoNode = static_cast<QSGVideoNode *>(oldNode);
 
+    QMutexLocker lock(&m_frameMutex);
+
     if (videoNode && videoNode->pixelFormat() != m_frame.pixelFormat()) {
 #ifdef DEBUG_VIDEOITEM
         qDebug() << "updatePaintNode: deleting old video node because frame format changed...";