1) Implemented anchoring of infill lines to perimeters with length

limited anchors, while before a full perimeter segment was always
   taken if possible.
2) Adapted the line infills (grid, stars, triangles, cubic) to 1).
   This also solves a long standing issue of these infills producing
   anchors for each sweep direction independently, thus possibly
   overlapping and overextruding, which was quite detrimental
   in narrow areas.
3) Refactored cubic adaptive infill anchroing algorithm
   for performance and clarity.

src/libslic3r/EdgeGrid.hpp

@@ -21,6 +21,7 @@ public:
 	void set_bbox(const BoundingBox &bbox) { m_bbox = bbox; }
 
 	void create(const Polygons &polygons, coord_t resolution);
+	void create(const std::vector<const Polygon*> &polygons, coord_t resolution);
 	void create(const std::vector<Points> &polygons, coord_t resolution);
 	void create(const ExPolygon &expoly, coord_t resolution);
 	void create(const ExPolygons &expolygons, coord_t resolution);