WMF: Recreate the MFMediaSession when loading a new media.

MFMediaSession doesn't seem to handle correctly the change of media
source, causing the playback not to work afterwards.
A single MFMediaSession was created and used for every loaded media, we
now create a new one whenever we load a new media (releasing the old one
beforehand).

Task-number: QTBUG-26819

Change-Id: Id99c9dd54e161823d9580933e063f16240806529
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>
Reviewed-by: Jason Barron <jason@cutehacks.com>

src/plugins/wmf/player/mfvideorenderercontrol.h

@@ -58,7 +58,7 @@ public:
     QAbstractVideoSurface *surface() const;
     void setSurface(QAbstractVideoSurface *surface);
 
-    IMFActivate* currentActivate() const;
+    IMFActivate* createActivate();
 
 protected:
     void customEvent(QEvent *event);
@@ -68,6 +68,8 @@ private Q_SLOTS:
     void present();
 
 private:
+    void clear();
+
     QAbstractVideoSurface *m_surface;
     IMFActivate *m_currentActivate;
     IMFSampleGrabberSinkCallback *m_callback;