Key contains pointers into DerivedConfig. Each surface has its own
DerivedConfig. This would cause memory corruption if the surface that
opened the initial font grid deallocates. Instead, let's make sure Key
owns its own DerivedConfig.
Fixes#1670
This fixes two issues:
- If no font families are set, font points would not change the hash.
- DPI changes would not change the hash.
When the hash doesn't change when it should, we reuse a font stack due
to changes in #1662. This would cause some font issues. :)
