[mythtv] Decision on FFmpeg repository

[Peter Bennett]

There were objections to using FFmpeg master. FFmpeg have now 
cherry-picked their mediacodec changes to the release branch for 4.0, 
which was the main reason for aiming for master. So at this point I 
think I should proceed with using the release branch rather than master.

The submodule approach I believe is unworkable because the submodule is 
not included in github tar downloads, so that would affect everybody who 
builds from those.

The subtree approach raises many concerns around bringing in 90,000 
extra commits and their logs. Also I am not sure whether starting a 
subtree on one FFmpeg release branch and then switching it to another 
FFmpeg release branch will work.

I am now inclined to get an FFmpeg repository of our own forked into the 
MythTV project, where we can merge the branch we are using with the 
latest changes from FFmpeg and our customizations. Incorporating the 
code into MythTV will be a copy over, as we have always done. This is 2c 
in my original email. It is not ideal but a bit better than what we have 
now.

We still have the option of changing course later if desired. I will 
make sure to document in the README.sync file, where the source was 
obtained and the relevant commit hashes that correspond in the private 
FFmpeg repository as well as in the main FFmpeg repository.

Peter.