changed the merge from using mkvmerge to using ffmpeg

[elisha464]

when merging the subtitles with the video mkvmerge was sometimes throwing warnings, and when it throws a warning the exit code of the program is 1 and that throws an error even though the merge is totally OK
I switched the merge to use ffmpeg, it feels much more suited for the job and it also removes the dependency on another executable

Checklist

[X] I've run npm run compile and it produce no error
[X] I've run npm run test and it produce no error
[X] I've not pushing more than one feature in that pull request
[X] My branch is updated with the latest from main when I make that pull request
[X] I've tested as much as I can my changes

[qlty-cloud-legacy]

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[Godzil]

The merge is not OK. Some episodes have incorrect subtitles, and they need to be corrected. Yes mkvmerge return with an error value for a warning; but the subtitles are really incorrect.

There is an open issue about that: #65, and instead of using a tool which does not warn/errored on invalid subtitles (some of the entries are beyond the length of the video) I would prefer to have a way to detect them and patch the file on the fly.

[elisha464]

are you sure that the problem is because of the subtitles? and if so what kind of fix did you have in mind?