[subset] Fix renumbering of (Chain)ContextFormat2

Jonathan says: "It was broken."

Lib/fontTools/subset.py

@@ -557,6 +557,7 @@ def __classify_context(self):
         self.Intersect = lambda glyphs, c, r: c.intersect_class(glyphs, r)
 
         self.ClassDef = 'InputClassDef' if Chain else 'ClassDef'
+        self.ClassDefIndex = 1 if Chain else 0
         self.Input = 'Input' if Chain else 'Class'
 
   if self.Format not in [1, 2, 3]:
@@ -676,12 +677,21 @@ def subset_glyphs(self, s):
   elif self.Format == 2:
     if not self.Coverage.subset(s.glyphs):
       return False
-    indices = getattr(self, c.ClassDef).subset(self.Coverage.glyphs,
-                                                 remap=False)
-    rss = getattr(self, c.RuleSet)
-    rss = [rss[i] for i in indices]
     ContextData = c.ContextData(self)
     klass_maps = [x.subset(s.glyphs, remap=True) for x in ContextData]
+
+    # Keep rulesets for class numbers that survived.
+    indices = klass_maps[c.ClassDefIndex]
+    rss = getattr(self, c.RuleSet)
+    rssCount = getattr(self, c.RuleSetCount)
+    rss = [rss[i] for i in indices if i < rssCount]
+    del rssCount
+    # Delete, but not renumber, unreachable rulesets.
+    indices = getattr(self, c.ClassDef).intersect(self.Coverage.glyphs)
+    rss = [rss if i in indices else None for i,rss in enumerate(rss)]
+    while rss and rss[-1] == None:
+      del rss[-1]
+
     for rs in rss:
       if not rs: continue
       ss = getattr(rs, c.Rule)
@@ -695,10 +705,6 @@ def subset_glyphs(self, s):
       for r in ss:
         c.SetRuleData(r, [[klass_map.index(k) for k in klist]
                for klass_map,klist in zip(klass_maps, c.RuleData(r))])
-    # Prune empty subrulesets
-    rss = [rs for rs in rss if rs and getattr(rs, c.Rule)]
-    setattr(self, c.RuleSet, rss)
-    setattr(self, c.RuleSetCount, len(rss))
     return bool(rss)
   elif self.Format == 3:
     return all(x.subset(s.glyphs) for x in c.RuleData(self))