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/TriangleMesh.hpp

@@ -23,6 +23,7 @@ class TriangleMesh
 public:
     TriangleMesh() : repaired(false) {}
     TriangleMesh(const Pointf3s &points, const std::vector<Vec3crd> &facets);
+    explicit TriangleMesh(const indexed_triangle_set &M);
 	void clear() { this->stl.clear(); this->its.clear(); this->repaired = false; }
     bool ReadSTLFile(const char* input_file) { return stl_open(&stl, input_file); }
     bool write_ascii(const char* output_file) { return stl_write_ascii(&this->stl, output_file, ""); }