Lay out chords and lyrics as segments

Lay out chords and lyrics as segments

Zero-width chords overlapped whenever the text under them, or even
empty: adjacent chords (intros/bridges without text), chords separated by
a single space, or a long chord name over a short syllable. The rule from
dc37808 only widened the second chord of a run, so the first pair still
collided.

Group each chord with the lyrics up to the next chord into a segment with
two rows. Its width is the wider of the two, which fixes the overlap.