select.lua: use sub-lines instead of forking to ffmpeg

This:

- Removes the dependency of ffmpeg in PATH - useful on Windows, macOS
  (?), Flatpak.
- Will support subrandr.
- Avoids redownloading network subtitles.
- Avoids slow demuxing of large mkvs.
- But no longer returns future lines of embedded subtitles.
- No longer returns embedded lines of regions you seeked beyond without
  playing them.
- Changing sub track drops cached subtitles.