INT 10 - VIDEO - READ LIGHT PEN POSITION (except VGA)
        AH = 04h
Return: AH = light pen trigger flag
            00h not down/triggered
            01h down/triggered
                DH,DL = row,column of character light pen is on
                CH = pixel row (graphics modes 04h-06h)
                CX = pixel row (graphics modes with >200 rows)
                BX = pixel column
Notes:  on a CGA, returned column numbers are always multiples of 2 (320-
          column modes) or 4 (640-column modes)
        returned row numbers are only accurate to two lines