INT 10 - VIDEO - PERFORM GRAY-SCALE SUMMING (VGA/MCGA)

	AX = 101Bh
	BX = starting palette register
	CX = number of registers to convert
Return: nothing
Desc:	convert the RGB values of one or more palette registers such that the
	  resulting values are grays with the same intensities as the original
	  colors
SeeAlso: AH=12h/BL=33h