Instead of only allowing for glyph indexes, allow ASCII characters as
replacements. So this glyph sequence
  glyphs: 65 8, 66 8, 67 8
Can be replaced by
  glyphs: "ABC"
provided that the glyph for "A", "B" and "C" are 65, 66 and 67
respectively and their advance is exactly 8.
x offset and y offset must always be 0 and every glyph must start a
cluster.