Instead, issue a `write_texture` for every glyph that isn't present in
the atlas. I think this should be fine, since cache eviction should be
rare and, therefore, uploads will be too.
The result here is lower memory usage for bigger caches.
I noticed that the `RecentlyUsed` map was actually a `RecentlyInserted`
map. This caused panics when trying to reduce the initial texture atlas size and
dynamically increase it (coming in another PR!).
