Fix regression causing thin walls to come out as zigzag paths. #1573 #1575

lib/Slic3r/ExPolygon.pm

@@ -203,6 +203,9 @@ sub _medial_axis_voronoi {
         }
     }
     
+    # apply Douglas-Peucker to straighten paths
+    @result = map $_->simplify($width / 7), @result;
+    
     return @result;
 }