V4 bonus

Sets up framework for doing v4 => v4 migrations for things we didn't catch in the first round. Use by running on an already migrated v4 style, e.g.

    gl-style-migrate already-on-v4.json

- [x] stops/linear/exponential => consolidated stops function migrations
- [x] 512 map tileSize migration (aka "z level off-by-1 migration")
- [x] text-max-angle migration

Others? cc @lbud @ansis