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/Fill/FillConcentricInternal.hpp

@@ -0,0 +1,26 @@
+#ifndef slic3r_FillConcentricInternal_hpp_
+#define slic3r_FillConcentricInternal_hpp_
+
+#include "FillBase.hpp"
+
+namespace Slic3r {
+
+class FillConcentricInternal : public Fill
+{
+public:
+    ~FillConcentricInternal() override = default;
+    void fill_surface_extrusion(const Surface *surface, const FillParams &params, ExtrusionEntitiesPtr &out) override;
+
+protected:
+    Fill* clone() const override { return new FillConcentricInternal(*this); };
+    bool no_sort() const override { return true; }
+
+    const PrintConfig       *print_config        = nullptr;
+    const PrintObjectConfig *print_object_config = nullptr;
+
+    friend class Layer;
+};
+
+} // namespace Slic3r
+
+#endif // slic3r_FillConcentricInternal_hpp_