Introduction of a greedy Traveling Salesman Problem algorithm,

producing better shortest path estimate than the "closest next neighbor"
heuristics. The new greedy algorithm utilizes KD tree for closest
end point search, and builds a graph to detect loops.

PerimeterGenerator newly uses the optimized TSP algorithm.

ExtrusionEntity has been refactored / simplified.

src/libslic3r/ShortestPath.hpp

@@ -0,0 +1,17 @@
+#ifndef slic3r_ShortestPath_hpp_
+#define slic3r_ShortestPath_hpp_
+
+#include "libslic3r.h"
+#include "ExtrusionEntity.hpp"
+#include "Point.hpp"
+
+#include <utility>
+#include <vector>
+
+namespace Slic3r {
+
+std::vector<std::pair<size_t, bool>> chain_extrusion_entities(std::vector<ExtrusionEntity*> &entities, const Point *start_near = nullptr);
+
+} // namespace Slic3r
+
+#endif /* slic3r_ShortestPath_hpp_ */