[varLib.cff] Fix merging bug when there are no blends or marking glyphs.

If the CFF2 VF has no blends, or no marking glyphs, the default variation table does not get built; this can be needed later for the PrivateDict.

Lib/fontTools/varLib/cff.py

@@ -161,7 +161,7 @@ def merge_PrivateDicts(top_dicts, vsindex_dict, var_model, fd_map):
 		For each key, step through each relevant source font Private dict, and
 		build a list of values to blend.
 	The 'relevant' source fonts are selected by first getting the right
-	submodel using model_keys[vsindex]. The indices of the
+	submodel using vsindex_dict[vsindex]. The indices of the
 	subModel.locations are mapped to source font list indices by
 	assuming the latter order is the same as the order of the
 	var_model.locations. I can then get the index of each subModel
@@ -180,7 +180,7 @@ def merge_PrivateDicts(top_dicts, vsindex_dict, var_model, fd_map):
 		# At the moment, no PrivateDict has a vsindex key, but let's support
 		# how it should work. See comment at end of
 		# merge_charstrings() - still need to optimize use of vsindex.
-		sub_model, model_keys = vsindex_dict[vsindex]
+		sub_model, _ = vsindex_dict[vsindex]
 		master_indices = []
 		for loc in sub_model.locations[1:]:
 			i = var_model.locations.index(loc) - 1
@@ -317,6 +317,19 @@ def _get_cs(charstrings, glyphName):
 		return None
 	return charstrings[glyphName]
 
+def _add_new_vsindex(model, key, masterSupports, vsindex_dict,
+		vsindex_by_key, varDataList):
+	varTupleIndexes = []
+	for support in model.supports[1:]:
+		if support not in masterSupports:
+			masterSupports.append(support)
+		varTupleIndexes.append(masterSupports.index(support))
+	var_data = varLib.builder.buildVarData(varTupleIndexes, None, False)
+	vsindex = len(vsindex_dict)
+	vsindex_by_key[key] = vsindex
+	vsindex_dict[vsindex] = (model, [key])
+	varDataList.append(var_data)
+	return vsindex
 
 def merge_charstrings(glyphOrder, num_masters, top_dicts, masterModel):
 
@@ -365,24 +378,24 @@ def merge_charstrings(glyphOrder, num_masters, top_dicts, masterModel):
 
 		# If the charstring required a new model, create
 		# a VarData table to go with, and set vsindex.
-		try:
 		key = tuple(v is not None for v in all_cs)
+		try:
 			vsindex = vsindex_by_key[key]
 		except KeyError:
-			varTupleIndexes = []
+			vsindex = _add_new_vsindex(model, key,masterSupports, vsindex_dict,
-			for support in model.supports[1:]:
+				vsindex_by_key, varDataList)
-				if support not in masterSupports:
-					masterSupports.append(support)
-				varTupleIndexes.append(masterSupports.index(support))
-			var_data = varLib.builder.buildVarData(varTupleIndexes, None, False)
-			vsindex = len(vsindex_dict)
-			vsindex_by_key[key] = vsindex
-			vsindex_dict[vsindex] = (model, [key])
-			varDataList.append(var_data)
 		# We do not need to check for an existing new_cs.private.vsindex,
 		# as we know it doesn't exist yet.
 		if vsindex != 0:
 			new_cs.program[:0] = [vsindex, 'vsindex']
+			
+	# If there is no variation in any of the charstrings, then vsindex_dict
+	# never gets built. This is could still be needed if there is variation 
+	# in the PrivatDict, so we will build the default data for vsindex = 0.
+	if not vsindex_dict:
+		key = (True)*num_masters
+		_add_new_vsindex(model, key, masterSupports, vsindex_dict,
+			vsindex_by_key, varDataList)
 	cvData = CVarData(varDataList=varDataList, masterSupports=masterSupports,
 						vsindex_dict=vsindex_dict)
 	# XXX To do: optimize use of vsindex between the PrivateDicts and