instancer: implement restricting axis ranges (L3)
instancer: implement restricting axis ranges (L3)

Added method to limitTupleVariationAxisRanges which takes a map of
axis tags to (min, max) ranges and drops entire deltasets when outside
of the new limits, or scales the ones that are within range.

Modified _TupleVarStoreAdapter to account for the fact that, when limiting
axes, existing regions can be modifed rathern than simply dropped
(see rebuildRegions).

Implemented limiting axis ranges for fvar, FeatureVariations, and avar.

Note how we pass user-scale coordinates to instantiateAvar, because we need
both the default normalized coordinates and the ones mapped forward (by
the very same avar table that we are instancing).

STAT table support will follow in a separate commit.

fonttools/fonttools,googlefonts/fonttools