Update reduce_color method to be a bit more precise and round value.

[lintbe]

trafficstars

As mentionned in Issue #50 there is an issue in color conversion in snes mode.

Here is a proposal fix.

[lintbe]

Here is an exemaple for color #34272a that convertted to #302028 and now converting to #382828

[Optiroc]

Superseded by #55