Gets the halftone as written, a halftone dictionary or stream (Types 1, 5, 6, 10 and 16), or null for the
device's default.
public CosObject? Halftone { get; }
ISO 32000-2 §10.6.5, Tables 127 to 132 (HT). Halftones are defined in device space (§10.6.1).