MSC: void far _setfillmask( unsigned char far *mask )

	- prototype in graph.h

	- mask =  8 by 8 bit fill mask array
	- sets the fill mask used to fill screen regions
	- any bit set to 1 will have the default color; any bit cleared
	  to zero leaves the pixel unchanged
	- when no fill mask is set (NULL), the default color only is used