aac_ac3_parser: simplify an expression.
ac3dec: simplify an expression.
Revert "ac3dec: simplify an expression."

This reverts commit cfb2af32cec35cef5b9deef8b00c3caa558474c4.
ac3dec: simplify some expressions.
Revert "aac_ac3_parser: simplify an expression."

This reverts commit 20ce6ca8a5698b2ee0eb3e114102a3e7b641480e.
aac_ac3_parser: simplify an expression.
aac_ac3_parser: implement request_channel_layout.
Cosmetics.
ac3dec: implement request_channel_layout.
aac_ac3_parser: wrap usage of AVCodecContext.request_channels in FF_API_REQUEST_CHANNELS.