[HarfBuzz] Following up with class=0

[Behdad Esfahbod]

The recent thread from Khaled plus a thread from my colleague cibu brought to
my attention that collect_glyphs() cannot really meaningfully work when glyphs
with class=0 are involved, since those will be all glyphs not mentioned in the
ClassDef table.  I don't think this is going to bite us seriously, but just
something to note.

-- 
behdad
http://behdad.org/