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/mfplayersession.h

@@ -178,6 +178,7 @@ private:
 
     SourceResolver  *m_sourceResolver;
     HANDLE           m_hCloseEvent;
+    bool m_closing;
 
     enum MediaType
     {