[Cin] Cutting mkv causes video to stop at cut points

[Andrea paz]

> Known issue?
I don't know how to answer it though CinGG has always had problems
with the mkv container. Some users have many problems others less so.
See, for example:
https://www.cinelerra-gg.org/bugtracker/view.php?id=137

Try using "mkvtoolnix" to rewrite the keyframes of the file by doing a
new encoding. It didn't actually work for me and the problems
remained.