Add a kDrawMode to make transparent pixels white or black

I have a font where the letters are outlined in black, and it would be convenient to be able to draw it with either a white fill, black fill, or keep it transparent. I can do the latter two options with kDrawModeFillBlack and kDrawModeCopy, but none of them seem to let me make transparent pixels white (kDrawModeFillWhite makes the outline white too). Maybe this isn't possible with how the OS loads images, but I'm not sure.

This would be especially useful when the font is drawn on top of a dithered background where both colors are interlaced. It makes the text more readable and acts as shortcut for drawing a white rect in the size of the font and then drawing the font over it.

We don't have a draw mode that does exactly that, but I bet you could achieve it with some combination of kDrawModeXOR/NXOR and kDrawModeInverted (possibly inverting the background first, drawing text, then inverting the result). There's usually a way!