gvr2 to support more than 50 axes

[davelab6]

fonttools/fonttools#2048 (comment) demonstrates a new gvar table is needed

[justvanrossum]

The title is misleading, as the limitation is not determined by the number of axes alone, but by (something like) num_axes × num_glyph_masters.

[justvanrossum]

You can have way more than 50 axes if you have way less than 1600 masters...

[behdad]

When we design gvar++ we should address a few other problems with that table as well. There's little sharing allow in that table right now.