[postprocessor/ffmpeg] Allow embedding subtitles in m4a

[ghost]

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Subtitles can be embedded in m4a same as mp4.

I imagine the reason m4a is not allowed now is because no one ever needed it or it is not video. However, given that webm may only contain audio, I think m4a should be allowed as well.

mpv shows subtitles fine for audio only files: on console, or on gui if thumbnail is embedded (or with --force-window=yes option).

[rautamiekka]

You can't embed subtitles into an audio codec, you need to create a separate vid to show the subtitles, at which point you no longer have an audio file but a vid file without anything but the text to see.

I guess FFmpeg does that automatically as a result of detecting a subtitle. Unless it uses some metadata field, I couldn't find any specs.

[Lesmiscore]

@rautamiekka Just like this one, audio files can have subtitles, and ffmpeg supports this. Formats for subtitles are very limited for m4a (which is same as mp4 except that m4a lacks video), but this should work.