brightness function
Implementation
RGBA brightness(RGBA color, num adj) {
adj = (adj > 1) ? 1 : adj;
adj = (adj < -1) ? -1 : adj;
adj = ~~(255 * adj).round();
return new RGBA(
red: clampPixel(color.red + (adj as int)),
green: clampPixel(color.green + adj),
blue: clampPixel(color.blue + adj),
alpha: color.alpha);
}