[subset] Fix up glyph closure
[subset] Fix up glyph closure

Followup fix from 13d48da3a91e7ac005f31b0f4dbca01cae7adbd7

When avoiding redoing the same lookup, we should only do that within
each run over all lookups.

Fixes issue where closing glyphs over NotoNastaliq was unreliable and
returning different results (which was caused by items in our frozenset's
being walked over in different orders).

Slows down closing over NotoNastaliq from 5s to 16s.  Speedup fixes coming
soon.

googlefonts/fonttools,fonttools/fonttools