Gets the font descriptors that override this one's metrics for classes of a CIDFont's glyphs, by class name (such as
Proportional or HKana, Table 123); empty when there are none. Entries that are not dictionaries, or that name a
font file, are left out.
public IReadOnlyDictionary<string, PdfFontDescriptor> ClassDescriptors { get; }
ISO 32000-2 §9.8.3.1, Table 122 (FD), and §9.8.3.3: each value holds metric entries only, overriding this descriptor's for
that class. A new dictionary of live views is built on each call.