Gets a value indicating whether optional content currently hides what is painted.
public bool IsHidden { get; }
ISO 32000-2 §8.11.3.1: inside a marked-content sequence tagged OC whose group or membership is off (in the states
OptionalContentState gives, by default those of the document's default configuration). Hidden
content still changes the graphics state and advances text; only its paint, glyph, image and shading events are withheld,
unless the processor asks for HiddenContent, which flags them instead.