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/xsp/Print.xsp

@@ -107,6 +107,7 @@ _constant()
     void set_step_started(PrintObjectStep step)
         %code%{ THIS->state.set_started(step); %};
     
+    void detect_surfaces_type();
     void process_external_surfaces();
     void discover_vertical_shells();
     void bridge_over_infill();