Looks up a code's notdef mapping (notdefchar, notdefrange), in this CMap and then its parents.
public bool TryGetNotdefCid(CharacterCode code, scoped out int cid)
A code read by ReadCode(ReadOnlySpan<Byte>).
The substitute CID; 0 when there is none.
ISO 32000-2 §9.7.6.3; Adobe TN 5014 §7.4 (beginnotdefchar, beginnotdefrange).