Bring back gst-libav for 3.32
I'm removing the gst-libav element from the 3.32 branch because it is really broken. After downgrading the branch from 1.15 (unstable, oops) to 1.14 (that's better), and removing all the patches, I realized tarball conversion was switching from the working Debian 1.15 snapshot to the busted upstream tarballs.
I don't get paid enough to put up with gst-libav so I'm going to release without it. We'll want to bring it back....