Consider automatically doing the offsetting in processLayer

[simoncozens]

Lots of filters start:

	params = {
		"S": { "offset": 0, ...

    def processLayer(self, thislayer, params):
        offsetpaths = self.saveOffsetPaths(
            thislayer, params["offset"], params["offset"], removeOverlap=True
        )
        outlinedata2 = setGlyphCoords(doAngularizzle(offsetpaths, 4))

Perhaps if processLayer sees that we have an offset member in the parameters, it should do the offsetting and store the result in instance variables. I'll have to look through and see if that's worthwhile - if there's enough commonality to make it worth abstracting, or if subtle things are different each time.