Revert "Don't make interpolatable interpolations"
Revert "Don't make interpolatable interpolations"

This reverts commit 828b17228e50d4a995c04fe4eb45fba1cd61705e. On
second thought, if someone really wants to do this, that's fine. It
probably won't happen accidentally.

googlei18n/fontmake,googlefonts/fontmake,googlei18n/fontmake,googlefonts/fontmake