more perimeters on overhangs: don't allow on top of holes

#46 #112

src/libslic3r/ExPolygon.hpp

@@ -80,6 +80,28 @@ inline size_t number_polygons(const ExPolygons &expolys)
     return n_polygons;
 }
 
+
+inline ExPolygon to_expolygon(const Polygon &other)
+{
+    ExPolygon ex;
+    ex.contour = other;
+    return ex;
+}
+
+inline ExPolygon to_expolygon(Polygon &&other)
+{
+    ExPolygon ex;
+    ex.contour = std::move(other);
+    return ex;
+}
+
+inline ExPolygons to_expolygon(const Polygons &other)
+{
+    ExPolygons exs;
+    for (const Polygon &po : other) exs.emplace_back(to_expolygon(po));
+    return exs;
+}
+
 inline Lines to_lines(const ExPolygon &src) 
 {
     size_t n_lines = src.contour.points.size();

src/libslic3r/PerimeterGenerator.cpp

@@ -293,7 +293,11 @@ void PerimeterGenerator::process()
             ExPolygons overhangs_unsupported;
             if (this->config->extra_perimeters && !last.empty()
                 && this->lower_slices != NULL  && !this->lower_slices->expolygons.empty()) {
-                overhangs_unsupported = diff_ex(last, this->lower_slices->expolygons);
+                //remove holes from lower layer, we only ant that for overhangs, not bridges!
+                ExPolygons lower_without_holes;
+                for (const ExPolygon &exp : this->lower_slices->expolygons)
+                    lower_without_holes.emplace_back(to_expolygon(exp.contour));
+                overhangs_unsupported = diff_ex(last, lower_without_holes);
                 if (!overhangs_unsupported.empty()) {
                     //only consider overhangs and let bridges alone
                     //only consider the part that can be bridged (really, by the bridge algorithm)