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.

- -

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