Takes into account whether there's pairing and if there's an LFE channel.
An SCE has more bits than CPE/2 since IS and M/S save quite a lot of bits
when channels are paired. And most of the SCEs we have are in surround
layouts which map it to the center channel, which usually carries all of
the dialogue and compression artifacts there are easily audiable.

Also refactors the init function a little bit and labels some parts of it.

Fixes bug #5233
Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>