Not optimized to use sortedness yet. Also start putting in place infra
to faster reject bad data.
A version of Chandas.ttf found on some Chrome bots has 660kb of GPOS,
mostly junk. That is causing 48 million of set->add() calls in
collect_glyphs(), which is insane.
In the upcoming commits, I'll be speeding that up by optimizing
add_sorted_array(), while also reducing work by rejecting out-of-sort
arrays quickly and propagate the rejection.
Part of https://bugs.chromium.org/p/chromium/issues/detail?id=794896
Behdad Esfahbod