Stop video filters emitting frames from the old position just

Stop video filters emitting frames from the old position just
after a seek.

If this happens, we get sequences like

1. emit some video at time 60s
2. seek to 40s
3. video frame emitted from filter at 60.1s; this sets _last_video_time
in player so that frames emitted between 40s and 60s are discarded;
hence the audio buffers overflow.

Should fix #1551.