def matte_replace(x, y)
      f = copy
      f.opacity = OpaqueOpacity unless f.alpha?
      target = f.pixel_color(x, y)
      f.transparent(target)
    end
def matte_replace(x, y)
      f = copy
      f.opacity = OpaqueOpacity unless f.alpha?
      target = f.pixel_color(x, y)
      f.transparent(target)
    end
Make transparent all pixels that are the same color as the
 pixel at (x, y).