[subset] Support GSUB/GPOS.FeatureVariations

We should add a test using the font at:
https://github.com/behdad/harfbuzz/issues/521

Test with:
$ fonttools subset avar_rvrn.ttf --text=r

Lib/fontTools/subset/__init__.py

@@ -1183,8 +1183,7 @@ def subset_lookups(self, lookup_indices):
 
 @_add_method(otTables.Lookup)
 def collect_lookups(self):
-    return _uniq_sort(sum((st.collect_lookups() for st in self.SubTable
+    return sum((st.collect_lookups() for st in self.SubTable if st), [])
-                           if st), []))
 
 @_add_method(otTables.Lookup)
 def may_have_non_1to1(self):
@@ -1220,6 +1219,7 @@ def neuter_lookups(self, lookup_indices):
 
 @_add_method(otTables.LookupList)
 def closure_lookups(self, lookup_indices):
+    """Returns sorted index of all lookups reachable from lookup_indices."""
     lookup_indices = _uniq_sort(lookup_indices)
     recurse = lookup_indices
     while True:
@@ -1235,6 +1235,7 @@ def closure_lookups(self, lookup_indices):
 
 @_add_method(otTables.Feature)
 def subset_lookups(self, lookup_indices):
+    """"Returns True if feature is non-empty afterwards."""
     self.LookupListIndex = [l for l in self.LookupListIndex
                             if l in lookup_indices]
     # Now map them.
@@ -1243,10 +1244,6 @@ def subset_lookups(self, lookup_indices):
     self.LookupCount = len(self.LookupListIndex)
     return self.LookupCount or self.FeatureParams
 
-@_add_method(otTables.Feature)
-def collect_lookups(self):
-    return self.LookupListIndex[:]
-
 @_add_method(otTables.FeatureList)
 def subset_lookups(self, lookup_indices):
     """Returns the indices of nonempty features."""
@@ -1254,17 +1251,15 @@ def subset_lookups(self, lookup_indices):
     # HarfBuzz chooses shaper for Khmer based on presence of this
     # feature.    See thread at:
     # http://lists.freedesktop.org/archives/harfbuzz/2012-November/002660.html
-    feature_indices = [i for i,f in enumerate(self.FeatureRecord)
+    return [i for i,f in enumerate(self.FeatureRecord)
-                       if (f.Feature.subset_lookups(lookup_indices) or
+            if (f.Feature.subset_lookups(lookup_indices) or
-                           f.FeatureTag == 'pref')]
+                f.FeatureTag == 'pref')]
-    self.subset_features(feature_indices)
-    return feature_indices
 
 @_add_method(otTables.FeatureList)
 def collect_lookups(self, feature_indices):
-    return _uniq_sort(sum((self.FeatureRecord[i].Feature.collect_lookups()
+    return sum((self.FeatureRecord[i].Feature.LookupListIndex
-                           for i in feature_indices
+                for i in feature_indices
-                           if i < self.FeatureCount), []))
+                if i < self.FeatureCount), [])
 
 @_add_method(otTables.FeatureList)
 def subset_features(self, feature_indices):
@@ -1273,6 +1268,41 @@ def subset_features(self, feature_indices):
     self.FeatureCount = len(self.FeatureRecord)
     return bool(self.FeatureCount)
 
+@_add_method(otTables.FeatureTableSubstitution)
+def subset_lookups(self, lookup_indices):
+    """Returns the indices of nonempty features."""
+    return [r.FeatureIndex for r in self.SubstitutionRecord
+            if r.Feature.subset_lookups(lookup_indices)]
+
+@_add_method(otTables.FeatureVariations)
+def subset_lookups(self, lookup_indices):
+    """Returns the indices of nonempty features."""
+    return sum((f.FeatureTableSubstitution.subset_lookups(lookup_indices)
+                for f in self.FeatureVariationRecord), [])
+
+@_add_method(otTables.FeatureVariations)
+def collect_lookups(self, feature_indices):
+    return sum((r.Feature.LookupListIndex
+                for vr in self.FeatureVariationRecord
+                for r in vr.FeatureTableSubstitution.SubstitutionRecord
+                if r.FeatureIndex in feature_indices), [])
+
+@_add_method(otTables.FeatureTableSubstitution)
+def subset_features(self, feature_indices):
+    self.ensureDecompiled()
+    self.SubstitutionRecord = [r for r in self.SubstitutionRecord
+                               if r.FeatureIndex in feature_indices]
+    self.SubstitutionCount = len(self.SubstitutionRecord)
+    return bool(self.SubstitutionCount)
+
+@_add_method(otTables.FeatureVariations)
+def subset_features(self, feature_indices):
+    self.ensureDecompiled()
+    self.FeaturVariationRecord = [r for r in self.FeatureVariationRecord
+                                  if r.FeatureTableSubstitution.subset_features(feature_indices)]
+    self.FeatureVariationCount = len(self.FeatureVariationRecord)
+    return bool(self.FeatureVariationCount)
+
 @_add_method(otTables.DefaultLangSys,
              otTables.LangSys)
 def subset_features(self, feature_indices):
@@ -1353,6 +1383,9 @@ def closure_glyphs(self, s):
         lookup_indices = self.table.FeatureList.collect_lookups(feature_indices)
     else:
         lookup_indices = []
+    if getattr(self.table, 'FeatureVariations', None):
+        lookup_indices += self.table.FeatureVariations.collect_lookups(feature_indices)
+    lookup_indices = _uniq_sort(lookup_indices)
     if self.table.LookupList:
         while True:
             orig_glyphs = frozenset(s.glyphs)
@@ -1396,6 +1429,13 @@ def subset_lookups(self, lookup_indices):
         feature_indices = self.table.FeatureList.subset_lookups(lookup_indices)
     else:
         feature_indices = []
+    if getattr(self.table, 'FeatureVariations', None):
+        feature_indices += self.table.FeatureVariations.subset_lookups(lookup_indices)
+    feature_indices = _uniq_sort(feature_indices)
+    if self.table.FeatureList:
+        self.table.FeatureList.subset_features(feature_indices)
+    if getattr(self.table, 'FeatureVariations', None):
+        self.table.FeatureVariations.subset_features(feature_indices)
     if self.table.ScriptList:
         self.table.ScriptList.subset_features(feature_indices, self.retain_empty_scripts())
 
@@ -1418,6 +1458,9 @@ def prune_lookups(self, remap=True):
         lookup_indices = self.table.FeatureList.collect_lookups(feature_indices)
     else:
         lookup_indices = []
+    if getattr(self.table, 'FeatureVariations', None):
+        lookup_indices += self.table.FeatureVariations.collect_lookups(feature_indices)
+    lookup_indices = _uniq_sort(lookup_indices)
     if self.table.LookupList:
         lookup_indices = self.table.LookupList.closure_lookups(lookup_indices)
     else:
@@ -1428,13 +1471,15 @@ def prune_lookups(self, remap=True):
         self.neuter_lookups(lookup_indices)
 
 @_add_method(ttLib.getTableClass('GSUB'),
-                         ttLib.getTableClass('GPOS'))
+             ttLib.getTableClass('GPOS'))
 def subset_feature_tags(self, feature_tags):
     if self.table.FeatureList:
         feature_indices = \
             [i for i,f in enumerate(self.table.FeatureList.FeatureRecord)
              if f.FeatureTag in feature_tags]
         self.table.FeatureList.subset_features(feature_indices)
+        if getattr(self.table, 'FeatureVariations', None):
+            self.table.FeatureVariations.subset_features(feature_indices)
     else:
         feature_indices = []
     if self.table.ScriptList:
@@ -1450,6 +1495,8 @@ def prune_features(self):
         feature_indices = []
     if self.table.FeatureList:
         self.table.FeatureList.subset_features(feature_indices)
+    if getattr(self.table, 'FeatureVariations', None):
+        self.table.FeatureVariations.subset_features(feature_indices)
     if self.table.ScriptList:
         self.table.ScriptList.subset_features(feature_indices, self.retain_empty_scripts())
 
@@ -1512,6 +1559,7 @@ def prune_post_subset(self, options):
     if not table.LookupList:
         table.FeatureList = None
 
+
     if table.FeatureList:
         self.remove_redundant_langsys()
         # Remove unreferenced features
@@ -1529,6 +1577,13 @@ def prune_post_subset(self, options):
     #if table.ScriptList and not table.ScriptList.ScriptRecord:
     #    table.ScriptList = None
 
+    if not table.FeatureList and hasattr(table, 'FeatureVariations'):
+        table.FeatureVariations = None
+
+    if hasattr(table, 'FeatureVariations') and not table.FeatureVariations:
+        if table.Version == 0x00010001:
+            table.Version = 0x00010000
+
     return True
 
 @_add_method(ttLib.getTableClass('GDEF'))