FFmpeg does not use ordered chapters

[TheAMM]

https://trac.ffmpeg.org/ticket/3123
This means MKVs with multiple segments will fail to generate thumbnails for the end of the file (because mpv figures the total duration and ffmpeg doesn't consider the other segments).
mpv handles them fine, but is slower in comparison.

Figure out a way to check if file uses ordered chapters and fall back to mpv
or start messing with parsing the segment list and concat filters 🤢

Or maybe invest into multiple worker threads and ditch ffmpeg.

[TheAMM]

Kind of fixed in 5c09b04 and 4b75fd3.