GltfSerializer: name hierarchy group nodes after their own element

With --element-hierarchy, the intermediate nodes representing spatial
structure (storey, building, site) were all named after whichever leaf
element triggered their creation, instead of their own entity. Use the
ancestor element at the current tree level, so --use-element-guids and
default naming both identify group nodes correctly.

Generated with the assistance of an AI coding tool.

src/serializers/GltfSerializer.cpp

@@ -249,7 +249,7 @@ void GltfSerializer::write(const IfcGeom::TriangulationElement* o) {
             size_t new_node_index = json_["nodes"].size();
             node_indices_[(*it)->product()] = new_node_index;
             node_array_.push_back(new_node_index);
-            parent_node["name"] = object_id(o);
+            parent_node["name"] = object_id(*it);
             parent_node["children"] = json::array({current_node_index});
             json_["nodes"].push_back(parent_node);