video: reset filters on seek

Drop queued frames on seek. Reset the internal state of some filters
that seem to need it as well: at least vf_divtc still produced some
frames using the previous PTS.

This fixes weird behavior with some filters on seeking. In particular,
this could lead to A/V desync or apparent lockups due to the PTS of
filtered frames being too far away from audio PTS.

This commit does only the minimally required work to fix these PTS
related issues. Some filters have state dependent on previously filtered
frames, and these are not automatically reset with this commit (even
vf_divtc and vf_softpulldown reset the PTS info only). Filters that
actually require a full reset can implement VFCTRL_SEEK_RESET.