fixed: Properly handle stacked files for subtitles

This is a followup for #4196 & #3819. Note that #4196 multi-sub implementation is pretty flawed and this should fix that.

Note that I'm not sure our current subtitle API is flexible/capable enough to properly handle future-support for stacked/multiple subtitles. For what we have, this seems to be the best approach, but of course any suggestions/discussions are welcome. And since we're still pre-Gotham-beta, this would be the time if we want to change our API after all.