Begins a tagged marked-content sequence that is a content item of element: allocates the page's next MCID,
links it into the structure tree and writes /S <</MCID n>> BDC, where S is the element's structure type.
public PdfMarkedContentReference BeginTaggedContent(PdfStructureElement element)
The structure element of this document the content belongs to.
ISO 32000-2 §14.6.1, §14.7.5.2 (MCIDs) and §14.7.5.4 (the parent tree). This is the one way to tag canvas content: the MCID is
allocated through AddMarkedContent(PdfPage), so the page gets its StructParents and the
parent tree its entry. End the sequence with EndMarkedContent(). An element drawn on several pages gets one content
item per sequence. MCIDs already in the page's content but not in the structure tree are not seen.