ImageData:mapPixel
Transform an image by applying a function to every pixel.
This function is a higher order function. It takes another function as a parameter, and calls it once for each pixel in the ImageData.
The passed function is called with six parameters for each pixel in turn. The parameters are numbers that represent the x and y coordinates of the pixel and its red, green, blue and alpha values. The function should return the new red, green, blue, and alpha values for that pixel. fu