Ranman's code was meant for B/W, but grayscale is another story
GrayGetPlane returns a pointer to the grayscale plane plane. Valid values for plane are LIGHT_PLANE and DARK_PLANE. To draw in black, draw in both planes.
Note: Do not assume that any plane is on 0x4C00 when in grayscale mode, due to hardware version 2 support. Also do not assume that the 2 grayscale planes are consecutive, this is not the case on hardware version 1.
Some software grayscale implementations have two consecutive grayscale planes, but the one in TIGCC and GCC4TI doesn't.