matroska-demux can't jump by gap in case of backward playback
Submitted by Farkas Levente
if we try to step forward in a matroska file then it's working even if the gap time is set. but when it's also set but i'd like to step backward then it's no longer working. the reason for this is in this line:
where you explicitly test only for positive rate while it'd have to implement for negative rate too.