pixel_drawing_mode is ignored for filled regular polygons (and other shapes?)

Reported by Samuel GOUGEON (@sgougeon)

Originally assigned to Calixte DENIZET

Bug description:

The common part of 2 polygons filled with different colors and partially 
overlaying is not colored according to the pixel_drawing_mode expected way.


How to reproduce the bug:

// Run the following lines. The common part is always blue whatever is the
// pixel_drawing_mode:

modes = ["and","andReverse","andInverted","clear","copy","copyInverted", ..
         "equiv", "invert", "noop", "nor", "nand", "or", "orReverse", ..
         "orInverted", "set", "xor" ]

for m = modes
    clf
    f = gcf()
    f.pixel_drawing_mode = m;  // setting the drawing mode
    
    plot2d([-1 1],[0 0])       // setting the axes
    a = gca();
    a.isoview = "on";
    xtitle(m)
    
    xrpoly([0 0],8,1,10)       // drawing a first filled shape
    e = gce();
    e.background = color("yellow");
    e.fill_mode = "on";
    
    xrpoly([0.5 0],8,1,10)  // drawing a 2nd filled shape with partial overlay
    e = gce();
    e.background = color("blue");
    e.fill_mode = "on";
    
    sleep(1000)
end