instancer: never drop STAT DesignAxes; only prune out-of-range AxisValues

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.

Lib/fontTools/varLib/instancer.py

@@ -981,62 +981,58 @@ def instantiateFvar(varfont, axisLimits):
 def instantiateSTAT(varfont, axisLimits):
     # 'axisLimits' dict must contain user-space (non-normalized) coordinates
 
-    # XXX do something with axisRanges
+    stat = varfont["STAT"].table
+    if not stat.DesignAxisRecord or not (
+        stat.AxisValueArray and stat.AxisValueArray.AxisValue
+    ):
+        return  # STAT table empty, nothing to do
+
     location, axisRanges = splitAxisLocationAndRanges(axisLimits, rangeType=AxisRange)
 
-    pinnedAxes = set(location.keys())
-
-    stat = varfont["STAT"].table
-    if not stat.DesignAxisRecord:
-        return  # skip empty STAT table
-
-    designAxes = stat.DesignAxisRecord.Axis
-    pinnedAxisIndices = {
-        i for i, axis in enumerate(designAxes) if axis.AxisTag in pinnedAxes
-    }
-
-    if len(pinnedAxisIndices) == len(designAxes):
-        log.info("Dropping STAT table")
-        del varfont["STAT"]
-        return
+    def isAxisValueOutsideLimits(axisTag, axisValue):
+        if axisTag in location and axisValue != location[axisTag]:
+            return True
+        elif axisTag in axisRanges:
+            axisRange = axisRanges[axisTag]
+            if axisValue < axisRange.minimum or axisValue > axisRange.maximum:
+                return True
+        return False
 
     log.info("Instantiating STAT table")
 
-    # only keep DesignAxis that were not instanced, and build a mapping from old
-    # to new axis indices
-    newDesignAxes = []
-    axisIndexMap = {}
-    for i, axis in enumerate(designAxes):
-        if i not in pinnedAxisIndices:
-            axisIndexMap[i] = len(newDesignAxes)
-            newDesignAxes.append(axis)
-
-    if stat.AxisValueArray and stat.AxisValueArray.AxisValue:
-        # drop all AxisValue tables that reference any of the pinned axes
+    # only keep AxisValues whose axis is not pinned nor restricted, or is pinned at the
+    # exact (nominal) value, or is restricted but the value is within the new range
+    designAxes = stat.DesignAxisRecord.Axis
     newAxisValueTables = []
     for axisValueTable in stat.AxisValueArray.AxisValue:
-            if axisValueTable.Format in (1, 2, 3):
-                if axisValueTable.AxisIndex in pinnedAxisIndices:
-                    continue
-                axisValueTable.AxisIndex = axisIndexMap[axisValueTable.AxisIndex]
-                newAxisValueTables.append(axisValueTable)
-            elif axisValueTable.Format == 4:
-                if any(
-                    rec.AxisIndex in pinnedAxisIndices
-                    for rec in axisValueTable.AxisValueRecord
-                ):
-                    continue
-                for rec in axisValueTable.AxisValueRecord:
-                    rec.AxisIndex = axisIndexMap[rec.AxisIndex]
-                newAxisValueTables.append(axisValueTable)
+        axisValueFormat = axisValueTable.Format
+        if axisValueFormat in (1, 2, 3):
+            axisTag = designAxes[axisValueTable.AxisIndex].AxisTag
+            if axisValueFormat == 2:
+                axisValue = axisValueTable.NominalValue
             else:
-                raise NotImplementedError(axisValueTable.Format)
+                axisValue = axisValueTable.Value
+            if isAxisValueOutsideLimits(axisTag, axisValue):
+                continue
+        elif axisValueFormat == 4:
+            # drop 'non-analytic' AxisValue if _any_ AxisValueRecord doesn't match
+            # the pinned location or is outside range
+            dropAxisValueTable = False
+            for rec in axisValueTable.AxisValueRecord:
+                axisTag = designAxes[rec.AxisIndex].AxisTag
+                axisValue = rec.Value
+                if isAxisValueOutsideLimits(axisTag, axisValue):
+                    dropAxisValueTable = True
+                    break
+            if dropAxisValueTable:
+                continue
+        else:
+            log.warn("Unknown AxisValue table format (%s); ignored", axisValueFormat)
+        newAxisValueTables.append(axisValueTable)
+
     stat.AxisValueArray.AxisValue = newAxisValueTables
     stat.AxisValueCount = len(stat.AxisValueArray.AxisValue)
 
-    stat.DesignAxisRecord.Axis[:] = newDesignAxes
-    stat.DesignAxisCount = len(stat.DesignAxisRecord.Axis)
-
 
 def getVariationNameIDs(varfont):
     used = []