[mythtv] Redone FFmpeg 4.4.1

[Klaas de Waal]

I just had this idea to look at the actual source code.

I was under the impression that both the old devel/ffmpeg-resync (now known
as devel/ffmpeg-resync-0) and the new devel/ffmpeg-resync were based on the
same FFmpeg version 4.4.1 and that the difference was in the way the resync
had been done. So I expected the source trees of both versions to be
identical.

However, I have now been looking at the code in FFmpeg/libavformat and I
see a number of differences.
It looks like at least some MythTV-specific code has been removed just like
that.
It also looks like the FFmpeg version is slightly different, as there are
also some changes in code that are not related to MythTV changes.
This in turn suggests to me that different FFmpeg versions (at least from
different commits) have been used.
At the same time, the version file libavcodec/version.h is identical so the
new code could have come from a commit not too far away in time.

This makes for me the current devel/ffmpeg-resync branch suspect and I
think we should not waste time in debugging this. Just discard it.

The way forward is to handle the following as separate tasks:
- Update FFmpeg version
- Develop a new workflow for creating a new MythTV version based on a
specific FFmpeg commit
- Reducing the number of MythTV-specific changes

The devel/ffmpeg-resync-0 that has been manually crafted by Peter uses the
FFmpeg 4.4.1 and works OK now so that is a good basis.
It looks to me that this is good enough to put in master. This then also
decouples the FFmpeg update to 4.4.1 from other changes.

The next step is to create a workflow that copies the FFmpeg code from a
specific commit and then applies all MythTV fixes.
This should then, in this stage, create a source tree that is identical to
what is now devel/ffmpeg-resync-0.
The check if the new source tree is identical to Peter's hand-crafted
version is then a good test for the new workflow.

When that has been done it is a good moment to investigate replacing the
MythTV extensions to the code one by one while at the same time keeping the
functionality intact without causing regressions.

Klaas.
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