Fixes https://github.com/prusa3d/Slic3r/issues/32#issuecomment-260135542

Reverts an inadverent bug introduced in 4460b5ce50

xs/src/libslic3r/PerimeterGenerator.cpp

@@ -120,17 +120,21 @@ PerimeterGenerator::process()
                     // from the line width of the infill?
                     coord_t distance = (i == 1) ? ext_pspacing2 : pspacing;
                     
-
-                    //FIXME Vojtech: Why there is a special case for the thin walls?
-                    // Gap fill is active all the time anyway and this is not the outer perimeter.
-//                    if (this->config->thin_walls) {
-                    if (false) {
+                    if (this->config->thin_walls) {
+                        // This path will ensure, that the perimeters do not overfill, as in 
+                        // prusa3d/Slic3r GH #32, but with the cost of rounding the perimeters
+                        // excessively, creating gaps, which then need to be filled in by the not very 
+                        // reliable gap fill algorithm.
+                        // Also the offset2(perimeter, -x, x) may sometimes lead to a perimeter, which is larger than
+                        // the original.
                         offsets = offset2(
                             last,
                             -(distance + min_spacing/2 - 1),
                             +(min_spacing/2 - 1)
                         );
                     } else {
+                        // If "detect thin walls" is not enabled, this paths will be entered, which 
+                        // leads to overflows, as in prusa3d/Slic3r GH #32
                         offsets = offset(
                             last,
                             -distance