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/src/texture_atlas.rs

@@ -1,7 +1,7 @@
 use ecolor::Color32;
 use emath::{Rect, remap_clamp};
 
-use crate::{AlphaFromCoverage, ColorImage, ImageDelta};
+use crate::{ColorImage, ImageDelta, TextOptions};
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 struct Rectu {
@@ -75,11 +75,11 @@ pub struct TextureAtlas {
     discs: Vec<PrerasterizedDisc>,
 
     /// Controls how to convert glyph coverage to alpha.
-    pub(crate) text_alpha_from_coverage: AlphaFromCoverage,
+    options: TextOptions,
 }
 
 impl TextureAtlas {
-    pub fn new(size: [usize; 2], text_alpha_from_coverage: AlphaFromCoverage) -> Self {
+    pub fn new(size: [usize; 2], options: TextOptions) -> Self {
         assert!(size[0] >= 1024, "Tiny texture atlas");
         let mut atlas = Self {
             image: ColorImage::filled(size, Color32::TRANSPARENT),
@@ -88,7 +88,7 @@ impl TextureAtlas {
             row_height: 0,
             overflowed: false,
             discs: vec![], // will be filled in below
-            text_alpha_from_coverage,
+            options,
         };
 
         // Make the top left pixel fully white for `WHITE_UV`, i.e. painting something with solid color:
@@ -121,7 +121,7 @@ impl TextureAtlas {
                     let coverage =
                         remap_clamp(distance_to_center, (r - 0.5)..=(r + 0.5), 1.0..=0.0);
                     image[((x as i32 + hw + dx) as usize, (y as i32 + hw + dy) as usize)] =
-                        text_alpha_from_coverage.color_from_coverage(coverage);
+                        options.alpha_from_coverage.color_from_coverage(coverage);
                 }
             }
             atlas.discs.push(PrerasterizedDisc {
@@ -138,6 +138,10 @@ impl TextureAtlas {
         atlas
     }
 
+    pub fn options(&self) -> &TextOptions {
+        &self.options
+    }
+
     pub fn size(&self) -> [usize; 2] {
         self.image.size
     }