libav: Use newer version of LIBAV. Patch by IRC user 'kvaster'.

source: http://pastebin.com/fGLBsdqb
Many thanks / all credit to @kvaster

[12:15] < kvaster> I've made a patch for newer libav locally. Will be able to submit today a bit later.
[12:17] < kvaster> http://pastebin.com/fGLBsdqb
[12:23] < kvaster> it was just small api changes inside libav, I'm worried only about audio stream buffer sizes.
[12:24] < kvaster> Anyway this is working for me at least.
Take any newer libav version, not just up to 56.0.0
Maintain compatibility for older versions of ffmpeg/libav which don't have the AV_ prefix on CODEC_ID_*