The idea of filling a path is sometimes clipping to a path. Here is a way to do your task with (almost) only MetaPost:

\startMPpage[offset=2mm]

path p, q ;

p := unitsquare scaled 4cm ;

z1 = point 3.6 of p ;

q := z1 -- urcorner p -- lrcorner p -- llcorner p -- cycle;

for i = 8 step -.5 until 0:

draw (i,0)*cm -- (i-4,4)*cm withpen pencircle scaled 2pt withcolor blue;

endfor;

clip currentpicture to q;

draw p ; draw q ;

label.llft("B", llcorner p) ;

label.lrt ("C", lrcorner p) ;

label.ulft("A", ulcorner p) ;

label.urt ("D", urcorner p) ;

label.lft ("M", z1) ;

\stopMPpage