Fix of Support interface is generated across whole print layer regardless if it's needed or not #4570

Disabled a suspicious piece of legacy code inherited from Slic3r,
which tries to fill in spots inside support interfaces. The code was
too aggressive and it was doing more harm than good.

src/libslic3r/SupportMaterial.cpp

@@ -572,6 +572,33 @@ void PrintObjectSupportMaterial::generate(PrintObject &object)
 
     BOOST_LOG_TRIVIAL(info) << "Support generator - Generating tool paths";
 
+#if 0 // #ifdef SLIC3R_DEBUG
+    {
+        size_t layer_id = 0;
+        for (int i = 0; i < int(layers_sorted.size());) {
+            // Find the last layer with roughly the same print_z, find the minimum layer height of all.
+            // Due to the floating point inaccuracies, the print_z may not be the same even if in theory they should.
+            int j = i + 1;
+            coordf_t zmax = layers_sorted[i]->print_z + EPSILON;
+            bool empty = true;
+            for (; j < layers_sorted.size() && layers_sorted[j]->print_z <= zmax; ++j)
+                if (!layers_sorted[j]->polygons.empty())
+                    empty = false;
+            if (!empty) {
+                export_print_z_polygons_to_svg(
+                    debug_out_path("support-%d-%lf-before.svg", iRun, layers_sorted[i]->print_z).c_str(),
+                    layers_sorted.data() + i, j - i);
+                export_print_z_polygons_and_extrusions_to_svg(
+                    debug_out_path("support-w-fills-%d-%lf-before.svg", iRun, layers_sorted[i]->print_z).c_str(),
+                    layers_sorted.data() + i, j - i,
+                    *object.support_layers()[layer_id]);
+                ++layer_id;
+            }
+            i = j;
+        }
+    }
+#endif /* SLIC3R_DEBUG */
+
     // Generate the actual toolpaths and save them into each layer.
     this->generate_toolpaths(object.support_layers(), raft_layers, bottom_contacts, top_contacts, intermediate_layers, interface_layers, base_interface_layers);
 
@@ -3657,21 +3684,23 @@ void PrintObjectSupportMaterial::generate_toolpaths(
                     top_contact_layer.merge(std::move(interface_layer));
             } 
 
+#if 0
             if ( ! interface_layer.empty() && ! base_layer.empty()) {
                 // turn base support into interface when it's contained in our holes
                 // (this way we get wider interface anchoring)
-                //FIXME one wants to fill in the inner most holes of the interfaces, not all the holes.
+                //FIXME The intention of the code below is unclear. One likely wanted to just merge small islands of base layers filling in the holes
+                // inside interface layers, but the code below fills just too much, see GH #4570
                 Polygons islands = top_level_islands(interface_layer.layer->polygons);
                 polygons_append(interface_layer.layer->polygons, intersection(base_layer.layer->polygons, islands));
                 base_layer.layer->polygons = diff(base_layer.layer->polygons, islands);
             }
+#endif
 
             // Top and bottom contacts, interface layers.
             for (size_t i = 0; i < 3; ++ i) {
                 MyLayerExtruded &layer_ex = (i == 0) ? top_contact_layer : (i == 1 ? bottom_contact_layer : interface_layer);
                 if (layer_ex.empty() || layer_ex.polygons_to_extrude().empty())
                     continue;
-                //FIXME When paralellizing, each thread shall have its own copy of the fillers.
                 bool interface_as_base = (&layer_ex == &interface_layer) && m_object_config->support_material_interface_layers.value == 0;
                 //FIXME Bottom interfaces are extruded with the briding flow. Some bridging layers have its height slightly reduced, therefore
                 // the bridging flow does not quite apply. Reduce the flow to area of an ellipse? (A = pi * a * b)