avar3, per glyph?

[davelab6]

Related to #150 I wondered if @simoncozens could make such a mergezilla font using avar2 instead of duplicate masters, but that would require avar2 to apply to glyph set ranges only. Possible in avar3?

[behdad]

I wrote in #150: "Per-glyph avar2" doesn't make sense to me. You already can do that: define whatever new axis you want in avar2, and use it only in a subset of glyphs.