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/Geometry.cpp

@@ -338,19 +338,19 @@ double rad2deg_dir(double angle)
     return rad2deg(angle);
 }
 
-Point circle_taubin_newton(const Points::const_iterator& input_begin, const Points::const_iterator& input_end, size_t cycles)
+Point circle_center_taubin_newton(const Points::const_iterator& input_begin, const Points::const_iterator& input_end, size_t cycles)
 {
     Vec2ds tmp;
     tmp.reserve(std::distance(input_begin, input_end));
     std::transform(input_begin, input_end, std::back_inserter(tmp), [] (const Point& in) { return unscale(in); } );
-    Vec2d center = circle_taubin_newton(tmp.cbegin(), tmp.end(), cycles);
+    Vec2d center = circle_center_taubin_newton(tmp.cbegin(), tmp.end(), cycles);
 	return Point::new_scale(center.x(), center.y());
 }
 
 /// Adapted from work in "Circular and Linear Regression: Fitting circles and lines by least squares", pg 126
 /// Returns a point corresponding to the center of a circle for which all of the points from input_begin to input_end
 /// lie on.
-Vec2d circle_taubin_newton(const Vec2ds::const_iterator& input_begin, const Vec2ds::const_iterator& input_end, size_t cycles)
+Vec2d circle_center_taubin_newton(const Vec2ds::const_iterator& input_begin, const Vec2ds::const_iterator& input_end, size_t cycles)
 {
     // calculate the centroid of the data set
     const Vec2d sum = std::accumulate(input_begin, input_end, Vec2d(0,0));