Support IMSC1 in HLS

[avelad]

IMSC1 in HLS

Carried as XML text inside fMP4 Segments
HLS defines support for the IMSC1 Text profile
Mark your Media Playlists as IMSC1 with the CODECS attribute
• CODECS="stpp.TTML.im1t, …"

//IMSC1 alongside WebVTT in a Master Playlist
#EXTM3U
#EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="vtt",LANGUAGE="eng",NAME="English",URI="vtt.m3u8"
#EXT-X-STREAM-INF:BANDWIDTH=90000,CODECS="avc1.4d001e,ac-3",SUBTITLES="vtt"
bipbop_gear1/prog_index.m3u8
#EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="imsc",LANGUAGE="eng",NAME="English",URI="imsc.m3u8"
#EXT-X-STREAM-INF:BANDWIDTH=90000,CODECS="avc1.4d001e,ac-3,stpp.TTML.im1t",SUBTITLES="imsc"
bipbop_gear1/prog_index.m3u8

Documentation:
https://devstreaming-cdn.apple.com/videos/wwdc/2017/504op4c3001w2f222/504/504_advances_in_http_live_streaming.pdf
https://developer.apple.com/videos/play/wwdc2017/504/
https://developer.apple.com/streaming/HLS-WWDC-2017-Preliminary-Spec.pdf

[joeyparrish]

We already support TTML in MP4 (independent of any particular manifest format such as HLS or DASH). The parser for TTML in MP4 is registered by MIME type and codec string, so we can simply add "stpp.TTML.im1t" to that registration:

https://github.com/google/shaka-player/blob/33c38e9c7ef47abb35b7bd27e374999ab5b64b22/lib/text/mp4_ttml_parser.js#L91

[ismena]

Small nit: hls parser currently assumes all text streams it encounters are vtt, so will need to update it as well.

[joeyparrish]

This turns out to be much more complicated than I realized. Still working on the solution. Thanks!

[joeyparrish]

Cherry-picked to v2.2.1