GP: Mask by Material

Currently, layers can mask other layers. My idea is to have an option to also mask layers by material instead of masking the whole layer.

Example use case is if I want to draw a character and make shadows affect only the skin and not his eyes, hair or clothes.

Or if you want to draw the iris on the white area of the eye. You could just make the eye material special instead of having three separate layers or more, which makes it very difficult to arrange depth. This way, I don't have to have a separate layer for the hair, eye mask, skin and the iris itself.