fix for high hole_brim_distance

When multiple parts were laid inside each other like russion dolls,
then some bad XOR voodoo would happen and some regions in between two parts would get fully filled with brim
rather than no brim being generated anywhere on the inside of parts.

src/SkirtBrim.cpp

@@ -236,10 +236,9 @@ Polygons SkirtBrim::getInternalHoleExclusionArea(const Polygons& outline, const
             Polygon hole_poly = part[hole_idx];
             hole_poly.reverse();
             Polygons disallowed_region = hole_poly.offset(10u).difference(hole_poly.offset( - line_widths[extruder_nr] / 2 - hole_brim_distance));
-            ret.add(disallowed_region);
+            ret = ret.unionPolygons(disallowed_region);
         }
     }
-    ret.unionPolygons();
     return ret;
 }