Fix all broken tests.

Thank you, Chris, for your initial port of this library.
I hope you find these changes useful.
I look forward to using this library, possibly for a 3D printer slicer written in native Go.

As it is written now, it runs 1000 random test cases plus 3 specific test cases that were particularly difficult to debug, but now are working.  When I remove all print statements, the tests run very quickly:
$ go test
PASS
ok  	github.com/gmlewis/go.clipper	0.034s