We don't need to cast to int when using the round function from py23,
as this is a backport of python3's built-in round and thus it returns
an int when called with a single argument.
Cleans up GDEF varstore as well.
What's left:
- In none of the varLib.merger Mergers we handle the CaretValueFormat3. That should be done,
even though no one uses this,
- GPOS/GSUB FeatureVariations are not applied. Shouldn't be hard.
- 'rvrn' should be folded into 'ccmp' or some other default feature.
