The size option is being misused to allow different
color combinations: Red/Cyan, Red/Blue, Red/Green, and Magenta/Cyan
(there are others we could add, if only someone could figure out the
math :-D)
Note - If Tux Paint is running in "Novice" complexity mode, no choices
will be available; it will use Red/Cyan, as before.
h/t O'Hare The Rabbit for the idea
Preview at 3x3 blockiness while dragging (to hopefully help
performance on slower systems / larger canvases; my Pixel 7 struggled
to keep up).
Render the full effect upon mouse button release.
