def each_pixel
      (0..@width-1).each do |x|
        (0..@height-1).each do |y|
          yield(x,y,get_pixel(x,y))
        end
      end
    end
def each_pixel
      (0..@width-1).each do |x|
        (0..@height-1).each do |y|
          yield(x,y,get_pixel(x,y))
        end
      end
    end
Walks each pixel on the image, block is mandantory to call this function and return
 as yield block calls x,y and pixel color for each pixel of the image

 This function does not allow to modify any color information of the image (use map_pixel for that)

 Examples:

   image.each_pixel do |x,y,color|
     print "pixel at (#{x},#{y}): #{color.inspect}\n"
   end