offset_converter should try to produce a closed output path if the input path is closed.
Currently it leaves the path open or produces self intersections and fails for certain offsets.
similar to #1339 (fix isn't working)
#1339 is probably unrelated. offset_converter doesn't handle SEG_CLOSE at all.
Other example, line-offset -5.
I think there should be some kind of switch in the code that treats closed ways differently from non-closed ways, if that's not already the case.
@springmeyer we should look at the code for http://www.angusj.com/delphi/clipper/documentation/Docs/Units/ClipperLib/Classes/ClipperOffset/_Body.htm to see if we could change what we do.