ffmpeg/libavresample: fixed 5.1 to stereo downmixing (there was no center channel).
Revert 657aacfa36, d8ca51c874, fdb28963f6, 2f08327770 commits.
Apply the patch by Aleksoid.
Revert "ffmpeg/libavresample: fixed 5.1 to stereo downmixing (there was no center channel)."

This reverts commit afaf886c1430b14d1cbe5cd4bba4261bab3c2941.

There was no proper review before pushing it.