I finally finished adding combining fills.
Except for the two basic cases, 'none', which simply replaces the target, and 'normal', which uses the alpha channel to weight the fill, I suspect they're mostly useful for special effects.
For example, if you wanted to turn an image into a rainbow, preserving the brightness, you could use the 'color' mode, which replaces the hue and saturation of the target pixel with that of the fill pixel.
I'll put a demo up soon.
