[alsa-devel] a52: update to new libavcodec API

Hi, this patchset updates the a52 plugin to work with the upcoming Libav 9 release. In doing that, it drops support for pre-0.8 libavcodec versions. I think it is not unreasonable to demand at least a year old libavcodec to compile latest git of alsa-plugins and keeping compatibility with the old audio encoding API would result in ugly #ifdefery mess.

The plugin compiles and appears to work, but as I have no SPDIF receiver, I cannot properly test it. I would very much appreciate it if someone else could do that.

Please comment.