load_layout: Correctly mark non-leaf containers

load_layout: Correctly mark non-leaf containers

Example problematic layout:
    {
        "layout": "splith",
        "marks": ["H1"],
        "nodes": [
            {
                "swallows": [
                    {
                        "class": "^a$"
                    }
                ]
            }
        ]
    }

Since the marks were added to the json_node during end_map, the
container that ended up getting the "H1" mark was the child instead of
the parent.