[FFmpeg-devel] Add support for LJ92 compressed MLV files, attempt 02

[South East]

ffmpeg has existing support for MLV raw video files; libavformat/mlvdec.c.
Since this was added, MLV has been extended to support LJ92 compressed
image data.  These patches build on lossless DNG support in 7.2-dev to
enable handling LJ92 MLV via existing libavcodec/mjpegdec.c code.

Sample LJ92 MLV file:
https://0x0.st/XlFm.mlv

FATE tests pass locally.  It was not obvious from the Developer guide how
to get this working.  The claim is "Running ’make fate’ accomplishes
[running local FATE tests]", but this of course does not work until
you rsync the samples, build FATE etc, which is not mentioned:
https://www.ffmpeg.org/developer.html#Regression-tests-1

I suggest adding something like "but WILL NOT work until it is configured",
before "please see fate.html for details".  This would make it clearer
that you NEED to follow the link, rather than it existing as reference
material should you want it in the future.  Simply running "make fate"
fails, but in a way that looked like a normal test failure to me;
I assumed upstream was broken since the failure was repeatable and
occured without my changes.

There is a long comment in patch 0001 that I don't expect to survive review,
but provides context for the code change.  As an ffmpeg noob I can't
make a good judgement on what level of commenting is appropriate.

For reference, attempt 1 can be seen in the archives, here:
https://ffmpeg.org//pipermail/ffmpeg-devel/2024-October/335245.html
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