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/PolylineStitcher.cpp

@@ -0,0 +1,42 @@
+//Copyright (c) 2022 Ultimaker B.V.
+//CuraEngine is released under the terms of the AGPLv3 or higher.
+
+#include "PolylineStitcher.hpp"
+#include "ExtrusionLine.hpp"
+
+namespace Slic3r::Arachne {
+
+template<> bool PolylineStitcher<VariableWidthLines, ExtrusionLine, ExtrusionJunction>::canReverse(const PathsPointIndex<VariableWidthLines> &ppi)
+{
+    if ((*ppi.polygons)[ppi.poly_idx].is_odd)
+        return true;
+    else
+        return false;
+}
+
+template<> bool PolylineStitcher<Polygons, Polygon, Point>::canReverse(const PathsPointIndex<Polygons> &)
+{
+    return true;
+}
+
+template<> bool PolylineStitcher<VariableWidthLines, ExtrusionLine, ExtrusionJunction>::canConnect(const ExtrusionLine &a, const ExtrusionLine &b)
+{
+    return a.is_odd == b.is_odd;
+}
+
+template<> bool PolylineStitcher<Polygons, Polygon, Point>::canConnect(const Polygon &, const Polygon &)
+{
+    return true;
+}
+
+template<> bool PolylineStitcher<VariableWidthLines, ExtrusionLine, ExtrusionJunction>::isOdd(const ExtrusionLine &line)
+{
+    return line.is_odd;
+}
+
+template<> bool PolylineStitcher<Polygons, Polygon, Point>::isOdd(const Polygon &)
+{
+    return false;
+}
+
+} // namespace Slic3r::Arachne