[opentype backend] Improve reading avar2 mapping strategy

[justvanrossum]

Discussion and code: Lorp/fencer#25

Short version: we need to take the StartCoord and EndCoord of each region into account as well, and then do some filtering.

[behdad]

I'm working on a fonttools counterpart here:
fonttools/fonttools#3598

I want to extend it to drop unnecessary pins.

[justvanrossum]

It would be great if you could factor it in a way so I can reuse it here.