pm3d clipping with respect to x, y coordinates of quadrangles can be done two
ways. clip1in: all 4 points of each quadrangle must be defined and at least
1 point of the quadrangle must lie in the x and y ranges. clip4in: all 4
points of each quadrangle must lie in the x and y ranges.
clipcb (default): palette color values 4#4 cmbin are clipped to cbmin.
noclipcb: quadrangles with palette color value 4#4 cbmin are not drawn at all.
