Cubics for glyf table discussion

[behdad]

Suggesting that we move cubics-for-glyf discussion here.

I see two ways:

• Use flag bit 7 to add cubics in, without bumping any version or major bump. glyf table has its version in head table's glyphDataFormat; not sure if any software checks that. That might be good if they don't check if we choose the semi-backward-compatible way (more about that below).

• Given that glyf table was the common thread we inherited from TrueType and has not changed in 30ish years, changing it drastically constitutes a new font format, so we might as well address many other issues at the same time, in particular: 1. 16bit GIDs, and 2. add versatile component reuse (including variations), particularly without consuming GIDs necessarily.

[behdad]

Closing in favor of commontype-standard/commontype#38