Use ne_map_track_number_to_index in nestegg_get_cue_point (from https://bugzilla.mozilla.org/show_bug.cgi?id=832018)

A track_number to track_id mapping was introduced in Mozilla Bug 804372 for nestegg. The patch did not update nestegg_get_cue_point, so the WebM files with track numbers other than 1 do not work in DASH.

New patch uses ne_map_track_number_to_index in nestegg_get_cue_point.