xvmc: Replace frame_started by picture_structure

xvmc: Replace frame_started by picture_structure

The preferred solution to keeping track of the picture structure
has been putting it in the state tracker, so use picture_structure
instead of frame_started to check if a frame needs to begin.

If picture_structure has been changed, end the frame and start again.

Signed-off-by: Maarten Lankhorst <m.b.lankhorst@gmail.com>
Signed-off-by: Christian König <deathsimple@vodafone.de>