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/CMakeLists.txt

@@ -183,6 +183,9 @@ add_library(libslic3r STATIC
     MinAreaBoundingBox.cpp
     miniz_extension.hpp
     miniz_extension.cpp
+    SimplifyMesh.hpp
+    SimplifyMeshImpl.hpp
+    SimplifyMesh.cpp
     ${OpenVDBUtils_SOURCES}
     SLA/Common.hpp
     SLA/Common.cpp