libslic3r core enhancements

* ClipperUtils extended with ExPolygon to clipper paths conversion and improved PolyTree traversal
* Added ExPolygon constructor with Polygon argument
* Removed BoundingBox warnings on clang
* Removed Geometry warnings on clang

src/libslic3r/Geometry.hpp

@@ -14,6 +14,11 @@
 using boost::polygon::voronoi_builder;
 using boost::polygon::voronoi_diagram;
 
+namespace ClipperLib {
+class PolyNode;
+using PolyNodes = std::vector<PolyNode*>;
+}
+
 namespace Slic3r { namespace Geometry {
 
 // Generic result of an orientation predicate.