Enable sharing across subtables of extension for harfbuzz packing.
Enable sharing across subtables of extension for harfbuzz packing.

Harfbuzz repacker can handle subtables shared across extension lookups so there's no need to disallow such sharing. Add a parameter to _doneWriting to allow sharing to be enabled when preparing the object graph for passing to harfbuzz.

googlefonts/fonttools,fonttools/fonttools