[WIP] [iOS & tvOS] Internal Text Subtitle Offset

[JPKribs]

Summary

Resolves: #1256 (comment)
Similar issue to: jellyfin/jellyfin-webos#85

I don't totally understand this. In my testing on the original fix (#1445) I guess I missed this because I was only using English subtitles and all of my internal subtitles are PGS. The issue appears to be that, when there are ONLY internal SUBRIP subtitles, the first and last subtitles have swapped DeliveryURLs. I do not understand WHY this is happening but I am able to recreate this consistently when testing with internal SUBRIP subtitle only. External Text-Based subtitles appear to work without issue. Internal Text-Based subtitles appear to work without issue IF there are external subtitles OR a graphic PGS subtitle included.

This logic switches the first and last subtitle tracks IF there are only text based subtitles internally. I will need to do some testing since I don't have a lot of files available to test this. Just getting this out there while I test this.

This PR also drops subtitles from transcoding where .isSupportsExternalStream is false since those are not selectable.

[JPKribs]

This is passing all my testing but I honestly have no idea WHY this happening unless there is something wonky with the API? Sorry to bother @LePips, but would you be able to test this out on your end and make sure I'm not missing anything? The fact this varies based on subtitle type makes be nervous I'm missing the bigger picture here...

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I think it might make more sense to look at this after: #1463

[JPKribs]

I'm closing this out until after #1463. I don't think that is going to resolve it but this issue is too inconsistent to properly replicate and test. Maybe the API changes resolve this but I'll keep an eye on this.