Make QGstreamerPlayerSession 'VAAPI-aware'.

Make QGstreamerPlayerSession 'VAAPI-aware'.

At the moment the GStreamer player pipeline is not setup properly when
the VAAPI plugin is available, resulting in no video being shown.

Added 'video/x-surface' as one of the default raw formats for the
decodebin.
Don't use vaapidecode when the video sink is not compatible.

This is a preliminary patch to support VAAPI. In the current state
vaapidecode will never be used as none of our video sinks support the
video/x-surface format.

Change-Id: I39f339b483d4052dd1e29c2b0ef06343d5670224
Reviewed-by: Christian Stromme <christian.stromme@digia.com>