Organic: Fix support blocker (#2595)

Expand blockers a little bit to compensate the error in facet projection

src/libslic3r/Support/TreeSupport.cpp

@@ -249,8 +249,12 @@ static std::vector<std::pair<TreeSupportSettings, std::vector<size_t>>> group_me
  raw_overhangs = overhangs;
  raw_overhangs_calculated = true;
  }
- if (! (enforced_layer || blockers_layers.empty() || blockers_layers[layer_id].empty()))
- overhangs = diff(overhangs, blockers_layers[layer_id], ApplySafetyOffset::Yes);
+ if (! (enforced_layer || blockers_layers.empty() || blockers_layers[layer_id].empty())) {
+ Polygons &blocker = blockers_layers[layer_id];
+ // Arthur: union_ is a must because after mirroring, the blocker polygons are in left-hand coordinates, ie clockwise,
+ // which are not valid polygons, and will be removed by offset. union_ can make these polygons right.
+ overhangs = diff(overhangs, offset(union_(blocker), scale_(g_config_tree_support_collision_resolution)), ApplySafetyOffset::Yes);
+ }
  if (config.bridge_no_support) {
  for (const LayerRegion *layerm : current_layer.regions())
  remove_bridges_from_contacts(print_config, lower_layer, *layerm,