WIP: Reworked the infill generator to merge areas with the same

properties.

Note for Vojtech:
Review src/libslic3r/Fill/Fill.cpp once again,
add test for G-code generator properties (extrusion speed, cooling?)

Fixes Modifier slice/move efficiency #1005

src/libslic3r/SurfaceCollection.hpp

@@ -37,6 +37,7 @@ public:
 
     void clear() { surfaces.clear(); }
     bool empty() const { return surfaces.empty(); }
+	size_t size() const { return surfaces.size(); }
     bool has(SurfaceType type) const { 
         for (const Surface &surface : this->surfaces) 
             if (surface.surface_type == type) return true;