Obtain designspace mappings from avar2 data

[Lorp]

Currently Fencer ignores any existing avar data, though it would be very useful if it could visualize existing avar1 and avar2 data as mappings.

Although a general solution may not be possible, it should be possible at least in all the cases where the avar2 table was compiled from mappings, which includes all fonts made via a .designspace document and all fonts exported from Fencer.

This code from Fontra is suggested by @justvanrossum, as noted in fonttools/fonttools#3086

https://github.com/googlefonts/fontra/blob/087fac4c01d5bbd918f3550d03fae5693f0dc292/src/fontra/backends/opentype.py#L255-L285