chromecast spu subtitles support

** Direct mux of subtitles into media ?

WebVTT seems the only option if we want to use mkv.

• WebVTT can only be muxed (using old deprecated mapping) into Webm by ffmpeg https://trac.ffmpeg.org/ticket/5434
• Webm ffmpeg mux makes VP8/9 codec and vorbis mandatory

Chromecast does not seem to "see" the embedded webvtt track. Maybe it needs to "enable" that track using a dedicated command.

"Your subtitle resources must implement CORS." https://developers.google.com/cast/docs/media So it seems all subtitles are split file.

** Side loaded subtitles ?

• We need the "command"
• There's no guarantee the chromecast does not expects a complete file when accessing it. (and the way WebVTT uses timestamps tends to make this mandatory)

We would need to extract/transcode all subtitles first.

The only option in that case seems to provide subtitles in segments (HLS, ...), which probably the whole media to be adaptive.

** Burn in subtitles ?

• Makes transcoding mandatory