Re: adding `NoColor`, I thought of that, but I don't think it's a very good solution.  If the SVG backend implements it by `fcA transparent` then we still have to worry about whether this is the same as no fill (it seems like it *should* be, of course, but I still don't know this for certain); and if other backends can't do that, then they might have to do some whole-tree analysis to figure out what to do---exactly what we are trying to avoid.