[request] Adding, using, handling 'mkvmerge' as middleware

[ner00]

Is your feature request related to a problem? Please describe. Sometimes I get MP4 videos that have some weird timing/sync issue. They work fine in some players, but they fail to play correctly with players like VLC or are even parsed incorrectly by platforms like WhatsApp.

Describe the solution you'd like The best way I've been able to deal with this issue is to use mkvmerge middleware from MKVToolNix to multiplex the file in an instant.

Describe alternatives you've considered All the attempts at fixing this with ffmpeg have failed or were successful but required reencoding the media file which added unnecessary time workload and strain to the CPU/GPU, and also ended up with output files much bigger than the input.

Additional context As an example: I sometimes download clips from X[.]com, I've run into this issue with every video downloaded from there, and while I may agree that the tools and/or platforms used to get the content might be the issue, I still rely on useful tools like FileConverter to come in handy every so often, even on edge cases like this.

While I don't exactly understand the strings attached to my request, I'd personally be satisfied if FC would allow for me to source the mkvmerge binary and just handle the interaction (supplied parameters, task status).

Thanks!