Gets the CMap that maps the font's character codes to CIDs: a built-in Identity CMap, a predefined CMap from the engine's
font resolvers (the Broadside.Fonts.Cmaps package, options.UsePredefinedCMaps()), or an embedded CMap stream. A
predefined CMap of Table 116 that no resolver supplies is recorded as unavailable and stood in for by a CMap with its
codespace, character collection and writing mode but no mappings (every code selects CID 0). When the Encoding entry
is missing, malformed, or names a CMap that is not in Table 116 and that no resolver supplies, codes are read as
IdentityH, with a diagnostic.
public CMap Encoding { get; }
ISO 32000-2 §9.7.5, §9.7.5.2 (Table 116) and §9.7.6.1, Table 119 (Encoding).