WIP: Splitting the number of top / bottom support interface layers.

If the new support_material_bottom_interface_layers is left at default -1,
then support_material_interface_layers is used for both top and bottom
interface layers.
If support_material_interface_layers == 0, then neither top nor bottom
interface layers are being extruded.

src/libslic3r/Flow.hpp

@@ -118,7 +118,10 @@ public:
 private:
     Flow(float width, float height, float spacing, float nozzle_diameter, bool bridge) : 
         m_width(width), m_height(height), m_spacing(spacing), m_nozzle_diameter(nozzle_diameter), m_bridge(bridge) 
-        { assert(width >= height); }
+        { 
+            // Gap fill violates this condition.
+            //assert(width >= height); 
+        }
 
     float       m_width { 0 };
     float       m_height { 0 };