ENH: increase grid infill's strength

Previously grid infill may be cut, so the strength on one direction is too weak. Now we switch the order at neighboring layers, so the strength of both directions are good enough.

Change-Id: I0ba768eeee34ce430e43ea57d8f06c209d469d39

src/libslic3r/Fill/FillBase.hpp

@@ -77,6 +77,7 @@ struct FillParams
     //BBS: only used for new top surface pattern
     float           no_extrusion_overlap{ 0.0 };
     bool            dont_sort{ false }; // do not sort the lines, just simply connect them
+    bool            can_reverse{true};
 };
 static_assert(IsTriviallyCopyable<FillParams>::value, "FillParams class is not POD (and it should be - see constructor).");