H.264 seeking is inaccurate

[itsmattkc]

This appears to be due to an underlying flaw in the FFmpeg API and leads to the symptoms described in #11 and #15 . When Olive tries to seek to a specific frame, it attempts to seek to the closest keyframe before the target frame and then play all frames up until that point to retrieve the target frame accurately. Unfortunately after seeking an H.264 video, FFmpeg appears to stay in the old location for 1-3 frames before actually performing the seek. This may affect other codecs too, but so far H.264 is the only codec that appears with the bug. This does not affect codecs with All-I frames (e.g. ProRes) and also does not appear to affect msvideo1.

[itsmattkc]

This bug can be seen especially clearly trying to step back frame-by-frame through an H.264 video. Olive does not step back smoothly. Instead it appears to alternate between playing the next frame and then stepping back.

[itsmattkc]

Turns out this was not an FFmpeg bug, this was caused by neglecting to flush the codec buffers before seeking. This has now been fixed as of 0a4832c