Nothing clever, if the source font has a CFF2 table use it as if it were
a CFF table (i.e. non-variable, not sure what would happen in the source
CFF2 was variable already).
I use the term 'location' for map of {axis_tag: float} coordinates, 'axisRanges'
for a map of {axis_tag: Tuple[float, float]} ranges, and 'axisLimits' to include
either single-float coordinates or range tuples.
Even if HVAR no longer contains any variations, it's better to keep it because
otherwise one would have to check the glyphs' phantom points to confirm that
the advance widths (or heights for VVAR) don't vary
The current method for L1 and L2 partial instacing of STAT table --
i.e. drop all pinned axes are respective axis values -- was incorrect.
STAT design axis are a superset of the fvar axes, they describe the relations
between members of a font family in which some aspects may be implemented as
variation axes within a single VF, others as multiple discrete fonts.
When we remove an axis from fvar, we still want to keep the STAT's DesignAxis,
as well as the single AxisValue table along that design axis which describes
the position of the new instance within the family's stylistic attributes.
This means, intantiateAvar will never drop any DesignAxis, but will only drops
AxisValue tables when: 1) we're pinning an axis and the desired instance
coordinate doesn't exactly equal any of the existing AxisValue records;
2) we're restricting an axis range, and the (nominal) AxisValue falls
outside of the desired range.
We never add new AxisValue records, as that's a design decision that
is outside of the scope of the partial instancer.
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.
