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.cpp

@@ -152,7 +152,9 @@ void FillGyroid::_fill_surface_single(
     ExPolygon                       &expolygon, 
     Polylines                       &polylines_out)
 {
-    expolygon.rotate(-this->angle);
+    float infill_angle = this->angle + (CorrectionAngle * 2*M_PI) / 360.;
+    if(abs(infill_angle) >= EPSILON)
+        expolygon.rotate(-infill_angle);
 
     BoundingBox bb = expolygon.contour.bounding_box();
     // Density adjusted to have a good %of weight.
@@ -228,9 +230,11 @@ void FillGyroid::_fill_surface_single(
     }
 
     // new paths must be rotated back
+    if(abs(infill_angle) >= EPSILON) {
         for (Polylines::iterator it = polylines_out.begin() + polylines_out_first_idx;
              it != polylines_out.end(); ++it) {
-        it->rotate(this->angle);
+            it->rotate(infill_angle);
+        }
     }
 }
 

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,