Implement difference with multiple clippings

[mfogel]

Currently there exists: difference(subjectGeom, clippingGeom)

This can be expanded to support multiple clippings:

difference(subjectGeom, clipGeom1, clipGeom2, ...)

Functionally this will be the same as difference(subjectGeom, union(clipGeom1, clipGeom2, ...)) just all done in one pass. It'll be more efficient, and will give difference the same function signature as the other operations.