re-wrote PrintObject::detect_surfaces_type() to C++,

Fixed some cracks in the fill surfaces created by rounding all surfaces inside detect_surface_type().

Fixed https://github.com/prusa3d/Slic3r/issues/12
Bridging-Angle not optimal

Extended the "Ensure veritcal wall thickness" mode (merged with the original discover_horizontal_shells function), but this a work in progress. Already Slic3r with "ensure vertical wall thickness" produces less spurious infills inside solids.

xs/src/libslic3r/Layer.cpp

@@ -75,12 +75,6 @@ Layer::clear_regions()
     this->regions.clear();
 }
 
-LayerRegion*
-Layer::get_region(int idx)
-{
-    return this->regions.at(idx);
-}
-
 LayerRegion*
 Layer::add_region(PrintRegion* print_region)
 {