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>