add support of WebVTT subtitles in Matroksa files

[Klaus Luppert]

I contacted Moritz Bunkus, the developer of MKVToolNix, because according to ironic_monkey from Kodi forum FFMPEG doesn't support the new spec. Here you can see the full thread at Kodi forum:
​http://forum.kodi.tv/showthread.php?tid=261529&pid=2309291

Moritz Bunkus explained the issue to me.
Up until a couple of days ago the storage of WebVTT in Matroska hadn't been specified yet. What ffmpeg seems to support (D_WEBVTT/…) is an old and outdated work-in-progress paper by the WebM project[1]. We (as in the Matroska project) have decided not to use their proposal in Matroska
for several reasons:

1. The proposal is old (from February 2012). The WebVTT specs have changed considerably since then, and therefore the WebM proposal doesn't handle several of those additions and changes.

2. It suggests splitting the single WebVTT file into multiple tracks inside WebM. This is possible but it makes handling of multiple tracks more difficult than it has to be for demuxers and remuxers. Additionally there's no way to specify that several tracks belong to the same logical track and must be handled all at once.

3. The WebM proposal is still a work in progress and WebM still officially doesn't support subtitles at all[2].

The Matroska project has now formalized the storage of WebVTT in
Matroska[3]. The specs have been added to the Matroska web site[4].

The next step should be to add support in ffmpeg for the current storage method.

[1] ​http://wiki.webmproject.org/webm-metadata/temporal-metadata/webvtt-in-webm
[2] ​http://www.webmproject.org/docs/container/
[3] ​http://thread.gmane.org/gmane.comp.multimedia.matroska.devel/5210
[4] ​https://www.matroska.org/technical/specs/subtitles/webvtt.html

Here you can get a sample file:
​https://drive.google.com/file/d/0B4y5u0gBKYqbSFczT1VPc2l5SXM/view?usp=sharing

[Klaus Luppert]

any news?