We should add an API on Path that takes the existing path, fills it with a given fill rule, takes a given Path, fills _that_ with a given fill rule, overlaps them, then adds all the segments of both Paths together into one Path (replacing the original Path), and removes all subsegments of that Path that are filled on both sides in the combined fill.

There's probably something else we need to do to make sure that the resulting path would fill in the same way, though, especially if we want to make sure it fills in the same way regardless of the fill rule.