ENH: add arachne engine for narrow internal solid infill

ConcentricGapFill pattern was used for internal narrow
solid infill. Use arachne engine instead to remove
gap fill inside the pattern and improve the extrusion path

Signed-off-by: salt.wei <salt.wei@bambulab.com>
Change-Id: I758d7c72eb71cc37026b7cebf746cc345014c3f5
(cherry picked from commit 0b6bacd21a091afc13d7b36a69e5b10f155bc6f8)

src/libslic3r/Arachne/utils/ExtrusionJunction.cpp

@@ -0,0 +1,18 @@
+//Copyright (c) 2020 Ultimaker B.V.
+//CuraEngine is released under the terms of the AGPLv3 or higher.
+
+#include "ExtrusionJunction.hpp"
+
+namespace Slic3r::Arachne
+{
+
+bool ExtrusionJunction::operator ==(const ExtrusionJunction& other) const
+{
+    return p == other.p
+        && w == other.w
+        && perimeter_index == other.perimeter_index;
+}
+
+ExtrusionJunction::ExtrusionJunction(const Point p, const coord_t w, const coord_t perimeter_index) : p(p), w(w), perimeter_index(perimeter_index) {}
+
+}