bugfix: don't insert Nef polyhedrons into the GeometryCache as it will compete with PolySets needed for OpenCSG rendering
This should fix Clipper-based non-cut projection
Implemented minkowski (2D and 3D)
ignore openscad_nogui
hull 2D and 3D implementation
Refactoring to facilitate more sharing of code between CGALEvaluator, GeometryEvaluator and CGALUtils
Don't convert empty clipper result to Polygon2d
Enforce x > 0 for rotate_extrude
Adapt to related changes in master
Clipper is bundled
Handle shared_ptr in signals/slots
Implemented 3D transform of PolySets, removed some Grid usage, improved PolySet -> Polyhedron conversion, optimized operations with only one child
Fixes remaining issues after merging #574
Updated test results with new behavior
Implemented resize
Upgraded ClipperLib to 6.1.2 - fixes a projection crash
Removed obsolete code
Use GeometryEvaluator instead of CGALEvaluator. A bunch of refactoring and fixes as a result of that. Renamed GUI menu items to reflect preview vs. render
Implemented export of Polygon2d to dxf
minor bugfixes after running tests