CTS-based seek is reasonable since player requests frames in output order
not coded order.

This change fixes seek to a keyframe within consecutive keyframes.
Let's say P[0|-1] and P[1|0], here x and y inside [x|y] are PTS and DTS
respectively, and both two frames are a keyframe. If you try to seek on
PTS=0, i.e. P[0|-1], you'll get P[1|0] if the demuxer is DTS based. This
is obviously undesirable.
Signed-off-by: Martin Storsjö <martin@martin.st>