* Speed up galley cache by only using the hash as key
This hashes the job but doesn't compare them with Eq,
which speeds up demo_with_tessellate__realistic by 5-6%,
winning back all the performance lost in
https://github.com/emilk/egui/pull/682
* Remove custom Eq/PartialEq code for LayoutJob and friends
* Silence clippy
* Unrelated clippy fixes
This PR introduces a completely rewritten text layout engine which is simpler and more powerful. It allows mixing different text styles (heading, body, etc) and formats (color, underlining, strikethrough, …) in the same layout pass, and baked into the same `Galley`.
This opens up the door to having a syntax-highlighed code editor, or a WYSIWYG markdown editor.
One major change is the color is now baked in at layout time. However, many widgets changes text color on hovered. But we need to do the text layout before we know if it is hovered. Therefor the painter has an option to override the text color of a galley.
## Performance
Text layout alone is about 20% slower, but a lot of that is because more tessellation is done upfront. Text tessellation is now a lot faster, but text layout + tessellation still lands at a net loss of 5-10% in performance. There are however a few tricks to speed it up (like using `smallvec`) which I am saving for later. Text layout is also cached, meaning that in most cases (when all text isn't changing each frame) text tessellation is actually more important (and that's more than 2x faster!).
Sadly, the actual text cache lookup is significantly slower (300ns -> 600ns). That's because the `TextLayoutJob` is a lot bigger (it has more options, like underlining, fonts etc), so it is slower to hash and compare. I have an idea how to speed this up, but I need to do some other work before I can implement that.
All in all, the performance impact on `demo_with_tesselate__realistic` is about 5-6% in the red. Not great; not terrible. The benefits are worth it, but I also think with some work I can get that down significantly, hopefully down to the old levels.
* Tweak style
More compact, less round, less noisy
* Button text is now same size as body text
* The rounder corners are now less rounded
* Collapsing headers no longer have a frame around them
* Combo-boxes looks better when opened
* Slightly more muted colors
* Remove extra line spacing after `\n` (i.e. between paragraphs)
* Thinner scrollbars
* Tweak light mode
* Tweak shadows
* Fix broken doc link
* Add style tweak to CHANGELOG
