Fixed some issues in internal anchors of the Adaptive Cubic infill.
The ugly and dangerous implicit casting operators in Line, MultiPoint,
Polyline and Polygon were made explicit.
BoundingBox constructor will no more throw for empty vector of points.
GMP allowed for Vojtech's fork of boost::polygon Voronoi implementation.
Added libslic3r tests for boost::polygon Voronoi. All Voronoi issues
ever reported on the Internet are captured by the tests. Two issues
reported (the two test cases) are real issues which may influence
PrusaSlicer negatively, namely
https://github.com/boostorg/polygon/issues/43
