INT 10H 12H BL=33H: Enable/Disable Gray-Scale Summing

                                                          Compatibility: VGA 
 Expects: AH    12H
          BL    33H
          AL    setting: 00H enable gray-scale summing
                         01H disable gray-scale summing
 Returns: AH    12H if a valid value was passed in AL
    Info: This updates bit 1 of the VgaFlagsRec (at 0040:0089).

          On the next mode reset (via INT 10H 00H), if that bit is set,
          then the BIOS will automatically reprogram the palette to display
          only shades of gray.

See Also: INT 10H 12H: EGA/VGA Special Functions/Alternate Select
          INT 10H: Video Services
          EGA/VGA Data Areas