Output seeking is too slow

[zmwangx]

In fdf3611 we fixed frame extraction in edge cases by introducing output seeking (i.e., -ss flag after -i, seeking frame by frame). This is way too slow, especially since each of the sixteen storyboard frames (by default) are sought from the beginning, so the whole video is roughly decoded eight times on average.

We might need to treat this case separately (I can't think of a good solution on top of my mind, except Create a thumbnail image every X seconds of the video, which requires a completely separate implementation of frame extraction (all frames must be extracted in a single pass). Since this is just an edge case, I'll deal with it if it turns out to be too annoying (this only occurs <1% of the time for me), or if there's request, or if I have too much time (highly unlikely).