Gets the box enclosing the figure's graphic on its page, in default user space: for a detected figure, its images, paths
and the text inside them, without the caption; for a tagged one, its element's BBox attribute, else the bounds of
its first extent. null when a tagged figure has neither.
public PdfRectangle? BoundingBox { get; }
ISO 32000-2 §14.8.5.4.3, Table 379 (BBox: required for Figure, Formula and Table in PDF/UA).