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

<!--
Please read the "Making a PR" section of
[`CONTRIBUTING.md`](https://github.com/emilk/egui/blob/main/CONTRIBUTING.md)
before opening a Pull Request!

* Keep your PR:s small and focused.
* The PR title is what ends up in the changelog, so make it descriptive!
* If applicable, add a screenshot or gif.
* If it is a non-trivial addition, consider adding a demo for it to
`egui_demo_lib`, or a new example.
* Do NOT open PR:s from your `master` branch, as that makes it hard for
maintainers to test and add commits to your PR.
* Remember to run `cargo fmt` and `cargo clippy`.
* Open the PR as a draft until you have self-reviewed it and run
`./scripts/check.sh`.
* When you have addressed a PR comment, mark it as resolved.

Please be patient! I will review your PR, but my time is limited!
-->

* Closes N/A
* [x] I have followed the instructions in the PR template

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/egui_demo_lib/tests/snapshots/tessellation_test/Additive rectangle.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:1a32d361afa20fc8c20122a89b01fe14b45849da42663991e589579f70fb8577
-size 46790
+oid sha256:a2b7b54a1af0f5cd31bd64f0506e3035dd423314ce3389e61730fa160434fbf3
+size 45074

crates/egui_demo_lib/tests/snapshots/tessellation_test/Blurred stroke.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:c8d55205cf4225123da33895ed45eb186e5e57184ef5928400a4bae3ab6092be
-size 88548
+oid sha256:7b66a0be67ff2d684a54c2321123521b3ad06dfe5ebffd50e89260d77efcfcc4
+size 86833

crates/egui_demo_lib/tests/snapshots/tessellation_test/Blurred.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:77ab9e2e18c788f8cdbec171269afe4d0a90c52abeda7063cae3766dcaa5e93b
-size 120612
+oid sha256:19320291c99a23429b114a59de4636689e281e1e68766abe2aa1e56562128e50
+size 118919

crates/egui_demo_lib/tests/snapshots/tessellation_test/Minimal rounding.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:36622a2f934503a7b60ded2f44b002e37eedde22d548dcf5a209f54c19548665
-size 53064
+oid sha256:5edf089c00715f1456fe7838e85aadcfc42b6216a3fd95b48d9c21fc8d700cba
+size 51371

crates/egui_demo_lib/tests/snapshots/tessellation_test/Normal.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:4adeb7a77a0d0fe85097fcd190a99b49858dce11bde601d30335afcb6cc3d5f6
-size 56276
+oid sha256:6cd1a10639dcb323bdc3b2c43e0c35665184fc809731ced90088ee9edb9de845
+size 54577

crates/egui_demo_lib/tests/snapshots/tessellation_test/Thick stroke, minimal rounding.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:f45249f7cc90433a64856905c727571c4ef20468e2c7d3ac2029e18a0477932d
-size 56743
+oid sha256:87e34024f701dc93f4026213ac7eb468a2cd6d3393eb0dbec382bf58007f8e61
+size 55042

crates/egui_demo_lib/tests/snapshots/tessellation_test/Thin filled.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:d437f68c521f3e627a1b50d46605e8b0b343c5fc3716d9669e8c4436c8992b6f
-size 37602
+oid sha256:d7940ff56796efb27bec66b632ff33aa2ad390c4962a711bf520aee341f035a4
+size 35968

crates/egui_demo_lib/tests/snapshots/tessellation_test/Thin stroked.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:08ba98437403a08cca825ed8e288c9f088a46d9a1081f0b0a4ed7fbb9b49bb85
-size 37640
+oid sha256:b7bbd16c8aad444f0d11aacf87cf2292d494cc80a1ca46e7e8db86ca3041d35a
+size 35931