Grainy surface marking

[Matthew Morrison]

As far as I'm aware, there is no way to mark surfaces on parts as "grainy", an effect most commonly known from 2x2 slope bricks. I think it would be good to have such thing, especially for being able to distinguish some possible clean and grainy variants (there are some iirc, but I might be wrong)

Cloth, fabric, and string parts would also benefit from some kind of texture flag. As it is, I just build a list of special parts and apply the appropriate material, but it's always a possibility a part will or won't be processed if the list is wrong.