Update subtitles management
Created by: nicosang
Hi,
this PR has to fix the issue #2084 (closed) : the index of the subtitles track is a reference of textTrackInfo array. textTrack array of html5 video element has a different index. In the same way, user can use the different subtitles with Firefox or Edge for instance. Indeed, before this PR, it was impossible to see subtitles in some specifc use cases (for instance, a first stream with one subtitles track, stop, load a second one, with two subtitles track).
Nicolas