Fix duplicate subtitles with different extensions

[asardaes]

Hi, I noticed some subtitle file duplicates after extraction, which happened because of the loop over the desired extensions:

• If srt was desired, every supported subtitle stream was extracted and saved with that extension.
• If ass was also desired, the same happened, overlapping with the subs extracted previously but now with a different extension.

I did a quick fix with a dict of extensions to supported codecs, but I only included the ones that still seem relevant, not sure what you think about that.

[klementng]

That's actually the intended behaviour. What I meant by 'desired' is the behaviour you are describing where all subtitles streams are extracted then saved/converted to the desired format. I intended it that way as I meant for this tool as an standardisation tool for subtitles (i.e. all in same srt format) regardless of the original format.

Perhaps what you're proposing or fixing could be a separate feature / flag where only targeted formats will extracted? I think it may be better implemented that way?

[asardaes]

But it's not really doing conversion, is it? At least I don't do any post-processing, and just extracting an ASS subtitle stream and saving it with an .srt extension results in a file that is not correctly recognized in the media player, I think.

[klementng]

There's conversation being done by ffmpeg during extraction process even without postprocessing. However, an possible issue I noticed in the past when converting ass to srt is that ffmpeg often leaves behind some positioning tags which I theorized is causing issues for some players in your case.

The postprocessing step for srt actually cleans it out and I personally never had any issues with player not recognizing the srt file afterwards.

The only issues with ass to srt is that some very complex ass files (i.e. multi-line texts) simply just can't be put in srt format without subtitles taking up the whole screen.

[asardaes]

Got it, then I think the behavior I added would need more changes to be a separate feature. I'll close this PR because it won't fit as it is then.