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.

tests/libslic3r/test_hollowing.cpp

@@ -9,6 +9,8 @@
 
 #include <libnest2d/tools/benchmark.h>
 
+#include <libslic3r/SimplifyMesh.hpp>
+
 #if defined(WIN32) || defined(_WIN32)
 #define PATH_SEPARATOR R"(\)"
 #else
@@ -23,6 +25,7 @@ static Slic3r::TriangleMesh load_model(const std::string &obj_filename)
     return mesh;
 }
 
+
 TEST_CASE("Negative 3D offset should produce smaller object.", "[Hollowing]")
 {
     Slic3r::TriangleMesh in_mesh = load_model("20mm_cube.obj");
@@ -40,3 +43,4 @@ TEST_CASE("Negative 3D offset should produce smaller object.", "[Hollowing]")
     in_mesh.require_shared_vertices();
     in_mesh.WriteOBJFile("merged_out.obj");
 }
+