Rockbox

This patch implements three new channel configurations
(Stereo Narrow, Stereo Wide, and Karaoke). This
fullfills feature request [622622] and more.

Stereo Narrow is implemented by partially mixing
the left and right channel together
(right = 0.75 * right + 0.25 * left).
This mode is usefull for listening those old mixes,
where left and right are completly separated, with
headphones.

Stereo Wide is implemented by subtracting part of
the left channel from the right channel and vice
versa (right = right - 0.5 left).
This mode can be used when listening through
speakers which are close together (e.g. computer
speakers). Whether the quality of the output
will be acceptable depends a lot on the actual mix.

Karaoke is implemented by completely subtracting
one channel from each other (right = right - left).
On mixes, where the voice comes fully centered from
the middle (most mixes), the voice disapeares
almost completely from the sound. The rest of the
sound can be very thin, if a lot of the instruments
are also comming from the center.

The patch also includes the bug fix patch
[686002] (channel config) simply because I don't
know how to create it separately with cvs.