Rewrote Fill2.pm to C++, deleted Perl infills for good.

Removed dependency on Perl Math::PlanePath module.
Fixed compilation with Visual Studio and SLIC3R_DEBUG: Visual Studio older than 2015 does not support the prinf type specifier %zu. Use %Iu instead.
C++11 move semantics enabled.

xs/src/libslic3r/Fill/FillBase.cpp

@@ -60,15 +60,25 @@ Polylines Fill::fill_surface(const Surface *surface, const FillParams &params)
 
 // Calculate a new spacing to fill width with possibly integer number of lines,
 // the first and last line being centered at the interval ends.
-//FIXME Vojtech: This 
 // This function possibly increases the spacing, never decreases, 
-// and for a narrow width the increase in spacing may become severe!
+// and for a narrow width the increase in spacing may become severe,
+// therefore the adjustment is limited to 20% increase.
 coord_t Fill::_adjust_solid_spacing(const coord_t width, const coord_t distance)
 {
-    coord_t number_of_intervals = coord_t(coordf_t(width) / coordf_t(distance));
-    return (number_of_intervals == 0) ? 
+    assert(width >= 0);
+    assert(distance > 0);
+    // floor(width / distance)
+    coord_t number_of_intervals = width / distance;
+    coord_t distance_new = (number_of_intervals == 0) ? 
         distance : 
         (width / number_of_intervals);
+    const coordf_t factor = coordf_t(distance_new) / coordf_t(distance);
+    assert(factor > 1. - 1e-5);
+    // How much could the extrusion width be increased? By 20%.
+    const coordf_t factor_max = 1.2;
+    if (factor > factor_max)
+        distance_new = coord_t(floor((coordf_t(distance) * factor_max + 0.5)));
+    return distance_new;
 }
 
 // Returns orientation of the infill and the reference point of the infill pattern.