C++11 build fix
Updated ClipperLib to 6.2.1
Compile fix: Don't include Carbon as it pollutes the global namespace with e.g. 'Polygon'
Moved Polygon typedef out of PolySet class
fixes crash when intersecting 3 or more 2d objects which results in an empty polygon
Added test for mixing empty 2D and 3D objects
Updated test results
Added test for dxf file not found
Added some nullspace tests
Added surface not found tests
Added tests for empty text results
createGeometry() should never return NULL
toPolygon2d() should never return NULL
doc
More correct handling of empty geometry. Should fix a few crash bugs
Added some malformed polyhedron tests and fixed an assertion bug
Added some malformed polygon tests
Added malformed offset tests
Update drawing logic (fixes #1035).

- Draw negative part of the axis with stippled lines
- Calculate contrast color for axis based on background color
- Disable lighting for axis / crosshair
Fix crosshair at the center of the display (fixes #1025).