[Bug 707032] qtdemux: add support for webvtt subtitles

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=707032
  GStreamer | gst-plugins-good | unspecified

--- Comment #9 from Andoni Morales <ylatuya at gmail.com> 2013-09-02 12:11:14 UTC ---
For webvtt in MP4 a sample would contain one VTTEmptyCueBox box (for a period
of no-zero duration without cue data) or one or more VTTCueBox with the same
start and stop time. The cue id goes in the 'iden' box, the cue point
configuration in the 'sttg' box and the text string in the 'payl' box.

For a cue point like:
1
00:00:10.000 --> 00:00:15.000 line:0 position:20% size:60% align:start
<b>First line.</b>

We would have a sample with:

sample PTS: 00:00:10.000 DUR: 00:00:5.000
 vttc
   iden=1
   sttg=line:0 position:20% size:60% align:start
   payl=<b>First line.</b>

It seems then appropriate to use subtitle/x-webvtt,variant={mp4,mpegts,raw} for
the different variants, where subtitle/x-webvtt,variant=mp4 would consist in
buffers with same timing info as the cue point, the payload as as data, and the
cue point configuration in the a GstBufferVTTMeta.

In case cue points overlaps and the duration of the sample is not
representative, the CueDurationBox should be used to set the real duration of
the cue point.

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