Also fixes issues with merging of PairPos.
Trying on Noto Sans Thai still fails, now because of issues in class differences in
PairPosFormat2. :( Investigating.
This is part of fixing https://github.com/fonttools/fonttools/issues/719
though, the changes are currently in interpolate_layout, and need to be ported /
merged with varLib.__init__ variation-font-builder.
The convention is that sys.exit(...) is called only if a module is run as a script,
and that main() entry points use return statements to report exit codes: 0 (or None)
for successful execution, or any non-zero integer for errors.
E.g. see the console scripts generated when installing with pip.
