Test that ligatures are always used, and only used, when they occur before the
glyphs for characters they are ligatures of. The word "fjord" is displayed twice;
the lower one must use the fj ligature and the upper one must not; the top of the letter f overlaps the
dot of the letter j..
Two fonts are defined, almost identical; in one the fj ligature is
defined after "f" but before "j"; in the second one it is the last glyph.
This tests longest-substring matching.
