[FFmpeg-trac] #9091(avformat:new): WEBVTT doesn't support magic-less metadata headers

[FFmpeg]

#9091: WEBVTT doesn't support magic-less metadata headers
----------------------------------+---------------------------------------
             Reporter:  mid-kid   |                     Type:  defect
               Status:  new       |                 Priority:  normal
            Component:  avformat  |                  Version:  unspecified
             Keywords:            |               Blocked By:
             Blocking:            |  Reproduced by developer:  0
Analyzed by developer:  0         |
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 FFmpeg version: 4.3.1

 WEBVTT is a weird format, but I just encountered a file that is written
 like:
 ```
 WEBVTT

 Region: id=r1 width=100% lines=3 regionanchor=0%,96%
 viewportanchor=0%,100% scroll=none
 Region: id=r2 width=100% lines=1 regionanchor=0%,96%
 viewportanchor=0%,100% scroll=none
 ....

 1
 00:00:01.240 --> 00:00:13.200 region:r1 line:88% align:left
 <c.white.background-black>#</c>
 ```

 FFmpeg interprets the block with the "Region:" metadata as a cue, due to
 being separated with two newlines from the "WEBVTT" magic, and the block
 not starting with "NOTE" (libavformat/webvttdec.c). This causes the
 subtitles not to show up in the MPV video player at all. Removing this
 extra newline between the magic and the region metadata solves the issue,
 but let's assume I can't modify these subtitle files.

 According to the spec[1], a webvtt file starts the magic, followed by
 optionally a tab or space, any additional characters, and *one or more
 line terminators*. This can be followed by zero or more metadata headers,
 which consist of a name, a colon, and a value.

 Unless I missed something, I believe these files are according to spec,
 and ffmpeg should be fixed to support this.

 [1]: https://dvcs.w3.org/hg/text-tracks/raw-
 file/default/608toVTT/region.html#extension-of-the-webvtt-file-body-syntax

--
Ticket URL: <https://trac.ffmpeg.org/ticket/9091>
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