Incomplete work for using Boost.Polygon to compute medial axis
More work to get Boost.Polygon to compile
Semi-working Boost.Polygon medial axis. Hangs after perimeters, though. Needs pruning and chaining
Finished implementing Boost.Polygon medial axis. Some cleanup needed
Fix failing XS test
Updated MANIFEST with Boost files
Merge branch 'master' into boost-medialaxis

Conflicts:
	lib/Slic3r/Layer/Region.pm
	xs/src/ExPolygon.cpp
	xs/src/Point.cpp
	xs/src/Point.hpp
	xs/src/TriangleMesh.cpp
	xs/t/01_trianglemesh.t
Rewritten half-edge traversal in a more robust way. Includes first medial axis unit test
More medial axis tests
Minor improvements to SVG debug output for medial axis
Promising approach to medial axis pruning
Consider contour thickness when validating medial axis segments
Add safety offset to thin wall regions to prevent overlapping geometry and corrupt Voronoi diagram
Fix the max width supplied to medial axis code: it's up to two extrusions
Some improvements to pruning and some additions here and there
Better pruning
Recursive pruning. Some more unit tests
Include thin walls in general top-level perimeter sorting to get more efficient paths
Initial implementation of linear gap fill
Better linear gap fill, now with adaptive extrusion width