Maximize gyroid printing speed angle

Counter-rotate the default angle by 45' so that gyroid is kept at it's
maximum printing speed by default.

src/libslic3r/Fill/FillGyroid.hpp

@@ -16,6 +16,11 @@ public:
     // require bridge flow since most of this pattern hangs in air
     virtual bool use_bridge_flow() const { return false; }
 
+    // Correction applied to regular infill angle to maximize printing
+    // speed in default configuration (degrees)
+    static constexpr float CorrectionAngle = -45.;
+
+
 protected:
     virtual void _fill_surface_single(
         const FillParams                &params,