Multiple tt:span per tt:p not supported - no subtitles displayed
Created by: davemevans
The tt:p element may have zero or more tt:span element(s) [1]. TTMLParser currently only considers the case where there are zero or one tt:span elements.
Test MPD is http://rdmedia.bbc.co.uk/dash/ondemand/elephants_dream/1/client_manifest-snake.mpd. This contains audio and video representations and a snaking subtitle representation. The subtitles in this are random text, but it demonstrates subtitles appearing word at a time.
[1] https://tech.ebu.ch/docs/tech/tech3380.pdf, 3.2.1.1.1