When running in sequence mode, we ignore the normal timestamps
of video and audio segments. This lead to problems in some Apple-
encoded webvtt content, which used the X-TIMESTAMP-MAP tag to account
for the timestamp offsets in their video. Thus, those subtitles would
end up 10 seconds offset.
This changes the webvtt parser to ignore the X-TIMESTAMP-MAP when in
sequence mode.
Issue #2337