avformat/webvttdec: Accept \r as newline

avformat/webvttdec: Accept \r as newline

After parsing the end timestamp of a WebVTT cue block, the current code
skips everything after the start of the timestamp that is not a \t, ' '
or \n and treats what is next as the start of a WebVTT cue settings list.
Yet if there is no such list, but a single \r, this will skip a part of
the cue payload (namely everything until the first occurence of \t, ' '
or \n) and treat what has not been skipped as the beginning of the
WebVTT cue settings list that extends until the next \r or \n (or the
end).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>