This does two things:
Fixes forced-set computation, which was wrong in multiple ways.
Debugged it. Is solid now... Famous last words.
Speeds up DP time by limiting DP lookback length. For Noto Sans,
IUP time drops from 23s down to 9s, with only a slight size increase
in the final font. This basically turns the algorithm from O(n^3) into
O(n).