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();
+}
+
+}