def matte_point(x, y)
      f = copy
      f.opacity = OpaqueOpacity unless f.alpha?
      pixel = f.pixel_color(x, y)
      pixel.opacity = TransparentOpacity
      f.pixel_color(x, y, pixel)
      f
    end
def matte_point(x, y)
      f = copy
      f.opacity = OpaqueOpacity unless f.alpha?
      pixel = f.pixel_color(x, y)
      pixel.opacity = TransparentOpacity
      f.pixel_color(x, y, pixel)
      f
    end
These four methods are equivalent to the Draw#matte method
 with the "Point", "Replace", "Floodfill", "FilltoBorder", and
 "Replace" arguments, respectively.
 Make the pixel at (x,y) transparent.