[varLib] Add VariationModel.getDeltasAndSupports()

which automatically fetches submodel.  Needs docstrings...

Part of https://github.com/googlei18n/fontmake/issues/88
Part of https://github.com/fonttools/fonttools/issues/1355

Lib/fontTools/varLib/__init__.py

@@ -411,9 +411,7 @@ def _merge_TTHinting(font, masterModel, master_ttfs, tolerance=0.5):
 	cvar.version = 1
 	cvar.variations = []
 
-	model, all_cvs = masterModel.getSubModel(all_cvs)
-	deltas = model.getDeltas(all_cvs)
-	supports = model.supports
+	deltas, supports = masterModel.getDeltasAndSupports(all_cvs)
 	for i,(delta,support) in enumerate(zip(deltas[1:], supports[1:])):
 		delta = [otRound(d) for d in delta]
 		if all(abs(v) <= tolerance for v in delta):

Lib/fontTools/varLib/models.py

@@ -338,6 +338,10 @@ class VariationModel(object):
 			out.append(delta)
 		return out
 
+	def getDeltasAndSupports(self, items):
+		model, items = getSubModel(items)
+		return model.getDeltas(items), model.supports
+
 	def getScalars(self, loc):
 		return [supportScalar(loc, support) for support in self.supports]