This fixes the performance problem we saw in
https://github.com/zed-industries/community/issues/2405.

In a trace we could see that if a font is missing we'd constantly look
it up and never cache that it's missing.

This changes that and does cache the font-is-missing result.

Drawback is that one would need to restart Zed after installing a
missing font that was configured in settings. That seems acceptable for
now, though.

Release Notes:

- Improved rendering performance when configured fonts are missing on
the system.