* Refactor text layout: don't need &Fonts in all functions
* Replace indexing in Fonts with member function
* Wrap Fonts in a Mutex
* Remove mutex for Font::glyph_info_cache
* Remove RwLock around Font::characters
* Put FontsImpl and GalleyCache behind the same Mutex
* Round font sizes to whole pixels before deduplicating them
* Make TextStyle !Copy
* Implement user-named TextStyle:s
* round font size earlier
* Cache fonts based on family and size
* Move TextStyle into egui and Style
* Remove body_text_style
* Query graphics about max texture size and use that as font atlas size
* Recreate texture atlas when it is getting full
* Move texture allocation into epaint/egui proper
* Add TextureHandle
* egui_glow: cast using bytemuck instead of unsafe code
* Optimize glium painter
* Optimize WebGL
* Add example of loading an image from file
This refactors the widget state storage introduced by @optozorax in https://github.com/emilk/egui/pull/257
* Unify the four buckets (`data`, `data_temp`, `id_data` and `id_data_temp`) into a single `data`.
* Less complexity, and also less chance of error (storing in one bucket, reading from another).
* Store data by `Id` and `TypeId`.
* Users can thus reuse the same `Id` to store many types.
* Uses a simple xor of id and typeid, which is fast and good since both id and typeid are already high-entropy hashes.
* Use different suffixes on the functions to pick if you want the data persisted or not (`get_temp`, `insert_persisted`, etc).
* Writing with one suffix and reading with the other works.
* To store state not bound to a specific `Id` (i.e. only based on type), use the new `Id::null` as the key.
