Duplicated text tracks

Created by: tomraut

At the moment there seems to be a bug in dash.js regarding how it handles subtitles. Both 2.4.1 and 2.5.0 versions doesn't seem to support the combination of text tracks specified through the stream manifest, and side loaded tracks.

The core problem seems to be that Dash.js addresses different text tracks by its own internal indexes, assuming that the video element doesn't contain any other entries than what's specified by itself. So when a text track is added from external file in addition to the one(s) included in the manifest, Dash.js later treats this as its own track, and add a duplicate set of cues (i.e. subtitle lines) to what's already loaded from the external file.