Add dithered layer modes

Hopefully that wasn’t posted before, but I feel a bunch of dithered layer mode would be super useful.

How would that work? Well, the opacity of the layer would be interpreted as a lookup toward your favorite dithering algorithm.

That way, you could shade with your good old semi-opaque brush technique while seeing a nondestructive dithering result.

I know about the brush pressure gradient feature, but I think it forces you into shading using pressure which, at least for me, feels quite unnatural.