[INTERNAL] IDL Z-buffer: POLYFILL alternative to TRIGRID

IDL Z-buffer: POLYFILL alternative to TRIGRID: try this instead of using TRIGRID and you will be pleasantly suprised.

   s=bytscl(stress,top=207)
   set_plot,'z'
   for i=0,1337 do polyfill,x(tri(*,i)),y(tri(*,i)),$
      color=s(tri(*,i))
   set_plot,'x' & tv,a
   set_plot,'z'
   a=tvrd()
   set_plot,'x' & tv,a