Syntax: DITHER x y min max fac

   Floyd-Steinberg dither the current IMAGE, resulting in the vectors of
points x and y; these are usually plotted as "ptype 1 1". Each pixel of
the image is represented by (up to) fac*fac points. Even fac == 1 can
produce recognisable images.

   Image values between min and max are used in the dithering; values outside
the range are taken to be min or max.

   Note that you can manipulate your image before dithering with
a command like
	set image[*,*] = (image[*,*] == 1234) ? 4321 : image[*,*]