[PROPOSAL] Validate TTML subtitles in ISO-BMFF

[donmartin00]

CCExtractor Version: 0.88

Necessary information

• Is this a regression (i.e. did it work before)? {NO}
• What platform did you use? {Window}
• What were the used arguments? {No arguments - we did not attempt to run this type of file through the gui because we did not see an option for the ISO-BMFF.}

Additional information

Add a feature to validate TTML subtitles in ISO-BMFF format per specification 14496-30, the format used in DVB Dash and ATSC 3.0).

[bubbaprog]

Not sure what method they're using to do it, but I just uploaded a raw ATSC 3.0 broadcast to Twitter and the captions appeared in the tweet— so a method for extracting and converting them is already out there in something presumably avconv/ffmpeg-related, & could possibly be used to develop it here.

[bubbaprog]

FWIW VLC can decode captions in ATSC3.0 video, and the code can likely be ported over. https://code.videolan.org/videolan/vlc/-/tree/master/modules/codec