Clear all previous track data for HTML-rendered tracks when disabling all tracks.
Created by: TobbeMobiTV
Fix for #804 (closed).
The player stops fetching data when no text track is visible, but the cues are not cleared in this case. The easiest way to reproduce the problem is to play content with HTML-rendered subtitles (like the reference player), and pause when subtitles are shown. When turning off the subtitle tracks, the text remains onscreen.