Need to reserve a slot in the color table for the transparent color
- before, if we were excluding the transparent color from the quantizer's input, it may very well generate 256 actually used colors
- then there'd be no opportunity to write the transparent color to the color table
- with this change, limit the quantizer's output to 255 if we're planning on writing the transparent color as well
- although, the quantizer has to be hacked a bit to work with potentially only 255 colors