Gets the structure element whose content the glyph is, when the page was read in structure order (the element holding the
marked-content sequence or object the glyph is shown in); null in content order and for artifacts that are
not in the structure tree.
public PdfStructureElement? StructureElement { get; }
ISO 32000-2 §14.7.5 (content items) and §14.7.5.4.