Gets the number of UTF-16 units of the glyph's text: usually one, two for a surrogate pair, more for a ligature, 0 for a code
mapped to no text or for a glyph whose text an ActualText replacement on the sequence's first glyph stands for.
public int TextLength { get; }
ISO 32000-2 §9.10.2 and §14.9.4.