[instancer.featureVars] Fix deficiency in L4 instancing

See discussion on the issue.  I think I got this right.

Fixes https://github.com/fonttools/fonttools/issues/2737

Lib/fontTools/varLib/instancer/featureVars.py

@@ -1,4 +1,5 @@
 
+from fontTools.ttLib.tables import otTables as ot
 from fontTools.varLib.models import normalizeValue
 from copy import deepcopy
 import logging
@@ -117,6 +118,7 @@ def _instantiateFeatureVariations(table, fvarAxes, axisLimits):
     featureVariationApplied = False
     uniqueRecords = set()
     newRecords = []
+    defaultsSubsts = None
 
     for i, record in enumerate(table.FeatureVariations.FeatureVariationRecord):
         applies, shouldKeep, universal = _instantiateFeatureVariationRecord(
@@ -128,7 +130,10 @@ def _instantiateFeatureVariations(table, fvarAxes, axisLimits):
 
         if applies and not featureVariationApplied:
             assert record.FeatureTableSubstitution.Version == 0x00010000
-            for rec in record.FeatureTableSubstitution.SubstitutionRecord:
+            defaultsSubsts = deepcopy(record.FeatureTableSubstitution)
+            for default,rec in zip(defaultsSubsts.SubstitutionRecord,
+                                   record.FeatureTableSubstitution.SubstitutionRecord):
+                default.Feature = deepcopy(table.FeatureList.FeatureRecord[rec.FeatureIndex].Feature)
                 table.FeatureList.FeatureRecord[rec.FeatureIndex].Feature = deepcopy(
                     rec.Feature
                 )
@@ -139,6 +144,16 @@ def _instantiateFeatureVariations(table, fvarAxes, axisLimits):
         if universal:
             break
 
+    # Insert a catch-all record to reinstate the old features if necessary
+    if featureVariationApplied and newRecords and not universal:
+        defaultRecord = ot.FeatureVariationRecord()
+        defaultRecord.ConditionSet = ot.ConditionSet()
+        defaultRecord.ConditionSet.ConditionTable = []
+        defaultRecord.ConditionSet.ConditionCount = 0
+        defaultRecord.FeatureTableSubstitution = defaultsSubsts
+
+        newRecords.append(defaultRecord)
+
     if newRecords:
         table.FeatureVariations.FeatureVariationRecord = newRecords
         table.FeatureVariations.FeatureVariationCount = len(newRecords)

Tests/varLib/instancer/instancer_test.py

@@ -1588,13 +1588,13 @@ class InstantiateFeatureVariationsTest(object):
         "location, appliedSubs, expectedRecords",
         [
             ({"wght": 0}, {}, [({"cntr": (0.75, 1.0)}, {"uni0041": "uni0061"})]),
-            # Buggy. See: https://github.com/fonttools/fonttools/issues/2737
             (
                 {"wght": -1.0},
                 {"uni0061": "uni0041"},
                 [
                     ({"cntr": (0, 0.25)}, {"uni0061": "uni0041"}),
                     ({"cntr": (0.75, 1.0)}, {"uni0041": "uni0061"}),
+                    ({}, {}),
                 ],
             ),
             (
@@ -1604,7 +1604,8 @@ class InstantiateFeatureVariationsTest(object):
                     (
                         {"cntr": (0.75, 1.0)},
                         {"uni0024": "uni0024.nostroke", "uni0041": "uni0061"},
-                    )
+                    ),
+                    ({}, {})
                 ],
             ),
             (
@@ -1622,7 +1623,8 @@ class InstantiateFeatureVariationsTest(object):
                     (
                         {"wght": (0.20886, 1.0)},
                         {"uni0024": "uni0024.nostroke", "uni0041": "uni0061"},
-                    )
+                    ),
+                    ({}, {})
                 ],
             ),
             (