WMF: more reliable video seeking.

When seeking, the presentation clock can enter an undefined state until
it is started again from the new position. Wait for the clock to be
restarted before scheduling the prerolled frames, otherwise these
frames might get a wrong presentation time.

Change-Id: I02cb3338239775b7ef5d206ec5aa1b26719ac978
Reviewed-by: Christian Stromme <christian.stromme@digia.com>