[mythtv] Resync to ffmpeg 4.0

[Peter Bennett]

I have re-synced the MythTV master source and put the results in branch 
devel/ffmpeg-resync to ffmpeg 4.0. I am now using the forked FFmpeg 
repository MythTV/FFmpeg to track our customization. Future merges 
should be easier, by creating the branch on MythTV/FFmpeg and doing a 
git merge from FFmpeg/FFmpeg. There could still be conflicts and errors 
needing fixing but I think the process will be an improvement.

Let me know if there are any problems with the devel/ffmpeg-resync branch.

Note the first commit caused a compile failure because I did not fix the 
plugins (I tend to forget those). That is why there are dozens of 
buildslave failures.

Peter