Improving POKEY hardware accuracy: non-linear mixing

[asiekierka]

The POKEY sound output doesn't mix the channels linearly; rather, it has a non-linear curve. This is required for more accurate-to-hardware reproduction.

Enabling this in the mzpokeysnd engine is trivial (you can enable it with #define NONLINEAR_MIXING). AltASAP might be more tricky.

I tried to find more authoritative information, but it's tricky...