add mesh simplification.

SPE-1072 
Working but flipped normals with the interior.
Testing on treefrog passed
Oversampling for hollowed mesh should not be less than 3x
Flip back normals after simplify and remove redundant test code.

src/libslic3r/SimplifyMesh.cpp

@@ -0,0 +1,66 @@
+#include "SimplifyMesh.hpp"
+#include "SimplifyMeshImpl.hpp"
+
+namespace SimplifyMesh {
+
+template<> struct vertex_traits<stl_vertex> {
+    using coord_type = float;
+    using compute_type = double;
+    
+    static inline float x(const stl_vertex &v) { return v.x(); }
+    static inline float& x(stl_vertex &v) { return v.x(); }
+    
+    static inline float y(const stl_vertex &v) { return v.y(); }
+    static inline float& y(stl_vertex &v) { return v.y(); }
+    
+    static inline float z(const stl_vertex &v) { return v.z(); }
+    static inline float& z(stl_vertex &v) { return v.z(); }
+};
+
+template<> struct mesh_traits<indexed_triangle_set> {
+    using vertex_t = stl_vertex;
+    static size_t face_count(const indexed_triangle_set &m)
+    {
+        return m.indices.size();
+    }
+    static size_t vertex_count(const indexed_triangle_set &m)
+    {
+        return m.vertices.size();
+    }
+    static vertex_t vertex(const indexed_triangle_set &m, size_t idx)
+    {
+        return m.vertices[idx];
+    }
+    static void vertex(indexed_triangle_set &m, size_t idx, const vertex_t &v)
+    {
+        m.vertices[idx] = v;
+    }
+    static Index3 triangle(const indexed_triangle_set &m, size_t idx)
+    {
+        std::array<size_t, 3> t;
+        for (size_t i = 0; i < 3; ++i) t[i] = size_t(m.indices[idx](int(i)));
+        return t;
+    }
+    static void triangle(indexed_triangle_set &m, size_t fidx, const Index3 &t)
+    {
+        auto &face = m.indices[fidx];
+        face(0) = int(t[0]); face(1) = int(t[1]); face(2) = int(t[2]);
+    }
+    static void update(indexed_triangle_set &m, size_t vc, size_t fc)
+    {
+        m.vertices.resize(vc);
+        m.indices.resize(fc);
+    }
+};
+
+} // namespace SimplifyMesh
+
+namespace Slic3r {
+
+void simplify_mesh(indexed_triangle_set &m)
+{
+    SimplifyMesh::implementation::SimplifiableMesh sm{&m};
+    sm.simplify_mesh_lossless();
+}
+
+}