Add color scaledown modes

[asiekierka]

From H-A-M-G-E-R:

And make it optional: provide several options:
simple right-shift, this, and multiplying by 31 then dividing by 256, rounded, floored or ceilinged

In particular, not having simple right-shift effectively causes a compatibility regression, and also some unintuitive behaviour (for example, on WonderSwan, the color #999999 now rounds up to #AAA, where most would expect it to stay at #999 as that's how emulators render it too).

I think "simple right-shift" (floor), "simple rounding" (round) and "multiplying by ((1 << shift) - 1) then dividing by 255" (scale) would be sufficient, with floor as the default to keep backwards compatibility.