Return Surface objects by reference from SurfaceCollection objects and fix a bug in XS code causing some shell options to be ignored

xs/src/Surface.cpp

@@ -0,0 +1,18 @@
+#include "Surface.hpp"
+
+namespace Slic3r {
+
+SV*
+Surface::to_SV_ref() {
+    SV* sv = newSV(0);
+    sv_setref_pv( sv, "Slic3r::Surface::Ref", (void*)this );
+    return sv;
+}
+
+double
+Surface::area() const
+{
+    return this->expolygon.area();
+}
+
+}

xs/src/Surface.hpp

@@ -16,6 +16,8 @@ class Surface
     unsigned short  thickness_layers;   // in layers
     double          bridge_angle;
     unsigned short  extra_perimeters;
+    SV* to_SV_ref();
+    double area() const;
 };
 
 typedef std::vector<Surface> Surfaces;