Replace ab_glyph with Skrifa + vello_cpu; enable font hinting (#7694)

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I'll probably come back to this and clean it up a bit. This PR
reimplements ab_glyph's functionality on top of Skrifa, a somewhat
lower-level font API that's being used in Chrome now.

Skrifa doesn't perform rasterization itself, so I'm using
[vello_cpu](https://github.com/linebender/vello) from the Linebender
project for rasterization. It's still in its early days, but I believe
it's already quite fast. It also supports color and gradient fills, so
color emoji support will be easier.

Skrifa also supports font hinting, which should make text look a bit
nicer / less blurry.

Here's the current ab_glyph rendering:

<img width="1592" height="1068" alt="image"
src="https://github.com/user-attachments/assets/2385b66e-23f8-4c6e-b8c2-ea90e0eea4e4"
/>

Here's Skrifa *without* hinting--it looks almost identical, but there
are some subpixel differences, probably due to rasterizer behavior:

<img width="1592" height="1068" alt="image"
src="https://github.com/user-attachments/assets/a815f3e9-65ac-4940-bc00-571177bef53d"
/>

Here's Skrifa  *with* hinting:

<img width="1592" height="1068" alt="image"
src="https://github.com/user-attachments/assets/d6cc0669-3537-4377-bba9-ed5ef09664db"
/>

Hinting does make the horizontal strokes look a bit bolder, which makes
me wonder once again about increasing the font weight from "light" to
"regular".

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/epaint/benches/benchmark.rs

@@ -1,8 +1,8 @@
 use criterion::{Criterion, criterion_group, criterion_main};
 
 use epaint::{
-    AlphaFromCoverage, ClippedShape, Color32, Mesh, PathStroke, Pos2, Rect, Shape, Stroke,
-    TessellationOptions, Tessellator, TextureAtlas, Vec2, pos2, tessellator::Path,
+    ClippedShape, Color32, Mesh, PathStroke, Pos2, Rect, Shape, Stroke, TessellationOptions,
+    Tessellator, TextureAtlas, Vec2, pos2, tessellator::Path,
 };
 
 use std::hint::black_box;
@@ -68,7 +68,7 @@ fn tessellate_circles(c: &mut Criterion) {
         let pixels_per_point = 2.0;
         let options = TessellationOptions::default();
 
-        let atlas = TextureAtlas::new([4096, 256], AlphaFromCoverage::default());
+        let atlas = TextureAtlas::new([4096, 256], Default::default());
         let font_tex_size = atlas.size();
         let prepared_discs = atlas.prepared_discs();