contrast function
Implementation
RGBA contrast(RGBA color, num adj) {
adj *= 255;
double factor = (259 * (adj + 255)) / (255 * (259 - adj));
return new RGBA(
red: clampPixel((factor * (color.red - 128) + 128).round()),
green: clampPixel((factor * (color.green - 128) + 128).round()),
blue: clampPixel((factor * (color.blue - 128) + 128).round()),
alpha: color.alpha,
);
}