gst player: clear the buffer pool when the video sink is stopped

Buffers were kept allocated until the player is destroyed or
next media played.

Change-Id: I8a0c85da3a82ac6883075a1d0674143783c7f010
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>

src/multimedia/gsttools_headers/qvideosurfacegstsink_p.h

@@ -94,6 +94,7 @@ public:
 
     QGstBufferPoolInterface *pool() { return m_pool; }
     QMutex *poolMutex() { return &m_poolMutex; }
+    void clearPoolBuffers();
 
     GstFlowReturn render(GstBuffer *buffer);