sd_ass contains some code that treats subtitle events with duration 0
specially, and adjust their duration so that they will disappear with
the next event.
This is most likely not needed anymore. Some subtitle formats allow
omitting the duration so that the event is visible until the next one,
but both subreader.c as well as libavformat subtitle demuxers already
handle this.
Subtitles embedded in mp4 files (movtext) used to trigger this code. But
these files appear to export subtitle duration correctly (at least
libavcodec's movtext decoder is using this assumption). Since commit
6dbedd2 changed demux_lavf to actually copy the packet duration field,
the code removed with this commit isn't needed anymore for correct
display of movtext subtitles. (The change in sd_movtext is for dropping
empty subtitle events, which would now be "displayed" - libavcodec does
the same.)
On the other hand, this code incorrectly displayed hidden events in .srt
subtitles. See for example the first event in SubRip_capability_tester.srt
(part of FFmpeg's FATE). These intentionally have a duration of 0, and
should not be displayed. (As of with this commit, they are still
displayed in external .srt subs because of subreader.c hacks.)
However, we can't be 100% sure that this code is really unneeded, so
just comment the code. Hopefully it can be removed if there are no
regressions after some weeks or months.
Add a basic infrastructure for subtitle converters. These converters
work sort-of like decoders, except that they produce packets instead
of subtitle bitmaps. They are put in front of actual decoders.
Start with sd_movtext. 4 lines of code are blown up to a 55 lines file,
but fortunately this is not going to be that bad for the following
converters.
