Fix for getting back to the same subtitle after turning them off.
Created by: TobbeMobiTV
Fixed issue #837 (closed) that it was impossible to turn off subtitles and getting back to the same language (for fragment-based subtitles).
Also unified with XML-file-based TTML subtitles and detecting TTML already from the codec/mime-type instead of from the cues. In all cases, if the video is paused as subtitles are shown, they vanish as the subtitles are turned off.