Int 10h, 12h /BL=33h    Select Grey-Scale Summing                   MCGA, VGA

    Selects whether the colors in the color registers will be averaged
    to grey scales when palette or color registers are loaded, e.g. on
    a mode set.

        Entry   AH = 12h
                AL = 0 | 1   Enable | disable summing
                BL = 33h

        Return  AL = 12h   If function supported