ffplay: use videoq.serial to detect obsolete frames

Previously VideoPicture->skip was used for that, using videoq.serial enables us
to detect obsolete frames right after pushing flush_pkt to the packet queue, so
we don't have to wait for the decoder thread to pop the flush packet anymore.

This is good because until now a full video picture queue may have blocked the
decoder thread.

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: flush video filters on seek

Actually we just re-create the filters, since there is no way to flush them.
Fixes hangup with yadif filter on seeking forward caused by a filtered frame
with a pts before the seek.

Signed-off-by: Marton Balint <cus@passwd.hu>
Merge remote-tracking branch 'cus/stable'

* cus/stable:
  ffplay: flush video filters on seek
  ffplay: use videoq.serial to detect obsolete frames

Merged-by: Michael Niedermayer <michaelni@gmx.at>