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.hpp

@@ -0,0 +1,25 @@
+#ifndef MESHSIMPLIFY_HPP
+#define MESHSIMPLIFY_HPP
+
+#include <vector>
+
+#include <libslic3r/TriangleMesh.hpp>
+
+namespace Slic3r {
+
+void simplify_mesh(indexed_triangle_set &);
+
+// TODO: (but this can be done with IGL as well)
+// void simplify_mesh(indexed_triangle_set &, int face_count, float agressiveness = 0.5f);
+
+template<class...Args> void simplify_mesh(TriangleMesh &m, Args &&...a)
+{
+    m.require_shared_vertices();
+    simplify_mesh(m.its, std::forward<Args>(a)...);
+    m = TriangleMesh{m.its};
+    m.require_shared_vertices();
+}
+
+} // namespace Slic3r
+
+#endif // MESHSIMPLIFY_H