Fix T84382: Geometry Nodes: Vertex group attributes are removed

In the report, the geometry is copied because it has two users and the
final join node needs to write to it. The join node also happens to
remove attributes apparently, because it exposed a mistake in the "copy"
method of the `MeshComponent` class. The copy is supposed to be
a deep copy, but the vertex group name map was not duplicated.

Differential Revision: https://developer.blender.org/D9991

source/blender/blenkernel/intern/geometry_set.cc

@@ -283,6 +283,7 @@ GeometryComponent *MeshComponent::copy() const
   if (mesh_ != nullptr) {
     new_component->mesh_ = BKE_mesh_copy_for_eval(mesh_, false);
     new_component->ownership_ = GeometryOwnershipType::Owned;
+    new_component->vertex_group_names_ = blender::Map(vertex_group_names_);
   }
   return new_component;
 }