function _sourceover() {
        or = sr;
        og = sg;
        ob = sb;
    }
This is the formula used by Photoshop to convert a color from
HSY (Hue, Saturation, Luminosity) to RGB (Red, Green, Blue).
The hue is calculated using the exacone approximation of the saturation
cone.
@param hsy The input color HSY normalized components.
@param rgb The output color RGB normalized components.