resampler: Fix confusion between rear and side channels for 5.1 layouts

resampler: Fix confusion between rear and side channels for 5.1 layouts

mpv and vlc play "normal" 5.1 AC3 and DTS files as if they had a
"5.1 (Side)" layout. Which is fine and consistent with ITU
recommendations if the user has a proper 7.1 system. But if the user
actually has a 5.1 system, PulseAudio will try to remap, poorly, between
the "5.1 (Side)" and "5.1" layouts, sending either an average between
front and rear channels, or an attenuated version of that average,
depending on the remixing-use-all-sink-channels setting.

This is not desired, the "Side" channels should be sent to "Rear", it is
only an unfortunate nomenclature confusion.

This patch does not fix 5.1 <-> 7.1 remixing.

Signed-off-by: Alexander E. Patrakov <patrakov@gmail.com>