Fragmented text bug fix
Created by: nicosang
Hi,
this PR has to solve two different issues about the management of fragmented subtitles. First issue can be reproduced by loading the stream https://livesim.dashif.org/dash/vod/testpic_2s/multi_subs.mpd. If the user seeks to a random position, and then change the language of the selected subtitle track, dash.js was downloading all the segments of the new track from the begining not from the current time. Second issue can also be reproduced, with Prince of Persia MSS stream. The user can't switch between two subtitles languages.
Nico