INT 10,4 - Read Light Pen Position

	AH = 04


	on return:
	AH = 0	light pen switch not triggered
	   = 1	light pen triggered
	BX = pixel column (0-319 or 0-639, mode dependent)
	CH = raster line (0-199) (CGA and EGA modes 4, 5 and 6)
	CX = raster line (EGA modes except 4, 5 and 6)
	DH = row (0-24)
	DL = column (0-79 or 0-79 mode dependent)


	- data returned as a byte coordinate, leaving horizontal
	  accuracy to within 2 pixels (320) or 4 pixels (640)
	- vertical accuracy within 2 lines
	- PS/2's don't support the light pen interface