Fix Fix triangle removal issues when using full narrow band of interior

src/libslic3r/OpenVDBUtils.cpp

@@ -68,6 +68,11 @@ openvdb::FloatGrid::Ptr mesh_to_grid(const indexed_triangle_set &    mesh,
         else if (subgrid) grid = std::move(subgrid);
     }
 
+    if (meshparts.size() > 1) {
+        // This is needed to avoid various artefacts on multipart meshes.
+        // TODO: replace with something faster
+        grid = openvdb::tools::levelSetRebuild(*grid, 0., 1.f, 1.f);
+    }
     if(meshparts.empty()) {
         // Splitting failed, fall back to hollow the original mesh
         grid = openvdb::tools::meshToVolume<openvdb::FloatGrid>(