Add some additional minkowski2d test cases
Fix a problem with the 2D Minkowski implementation

Polygons were clipped too soon, resulting in numerical robustness errors
which could create cracks in the resulting geometry. This patch corrects
the problem by postponing Clipper's union operation until the Minkowski
insides are filled. The result is also a net reduction of the number
of Clipper operations.
Merge pull request #611 from OskarLinde/minkowski2_bug

Minkowski2 bug