AERemap: Fix mixing front and rear channels in some 5.1 setups

Fix the case when the input announces BR and BL instead of SR and SL,
e.g. when the following channels are in:
input:  FL FR FC LFE SL SR
output: FL FR BL BR  FC LFE
This leads to wrong mix front and rear channels,
(e.g. when playing AC3 test files).
The commit is handling those cases.

Special thanks to @fritsch for helping to track the problem down
and initial test patches.
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