fonts: Implement CSS font-variation-settings property for FreeType platforms (#38642)

This change adds support for variable fonts via the
[`font-variation-settings`](https://developer.mozilla.org/en-US/docs/Web/CSS/font-variation-settings)
property.

There are three areas where we need to set the variation values:
* Webrender (`compositor.rs`), for drawing the glyphs
* Harfbuzz (`shaper.rs`), for most shaping tasks
* PlatformFont (`fonts/platform/`), for horizontal advances and kerning

For now, freetype is the only platform shaper that supports variable
fonts. I can't easily test the fonts with non-freetype shapers. Thats
why variable fonts are behind the `layout_variable_fonts_enabled` pref,
which is disabled by default.

<img width="1250" height="710" alt="image"
src="https://github.com/user-attachments/assets/1aee1407-f3a2-42f6-a106-af0443fcd588"
/>

<details><summary>HTML test file</summary>

```html
<style>
@font-face {
  font-family: "Amstelvar VF";
  src: url("https://mdn.github.io/shared-assets/fonts/variable-fonts/AmstelvarAlpha-VF.woff2")
    format("woff2-variations");
  font-weight: 300 900;
  font-stretch: 35% 100%;
  font-style: normal;
  font-display: swap;
}

p {
  font:
    1.2em "Amstelvar VF",
    Georgia,
    serif;
  font-size: 4rem;
  margin: 1rem;
  display: inline-block;
}

.p1 {
  font-variation-settings: "wght" 300;
}

.p2 {
  font-variation-settings: "wght" 625;
}

.p3 {
  font-variation-settings: "wght" 900;
}

</style>
<div>
  <p class="p1">Weight</p>
  <span>(font-variation-settings: "wght" 300)</span>
</div>
<div>
  <p class="p2">Weight</p>
  <span>(font-variation-settings: "wght" 625)</span>
</div>
<div>
  <p class="p3">Weight</p>
  <span>(font-variation-settings: "wght" 900)</span>
</div>
</div>
```
</details>



https://github.com/user-attachments/assets/9e21101a-796a-49fe-b82c-8999d8fa9ee1


Testing: Needs decision on whether we want to enable the pref in CI
Works towards https://github.com/servo/servo/issues/37236

Depends on https://github.com/servo/stylo/pull/230

---------

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

components/fonts/system_font_service.rs

@@ -28,7 +28,7 @@ use style::values::computed::font::{
 };
 use style::values::computed::{FontStretch, FontWeight};
 use style::values::specified::FontStretch as SpecifiedFontStretch;
-use webrender_api::{FontInstanceFlags, FontInstanceKey, FontKey};
+use webrender_api::{FontInstanceFlags, FontInstanceKey, FontKey, FontVariation};
 
 use crate::font::FontDescriptor;
 use crate::font_store::FontStore;
@@ -65,6 +65,7 @@ pub enum SystemFontServiceMessage {
         FontIdentifier,
         Au,
         FontInstanceFlags,
+        Vec<FontVariation>,
         IpcSender<FontInstanceKey>,
     ),
     GetFontKey(IpcSender<FontKey>),
@@ -94,7 +95,7 @@ pub struct SystemFontService {
     local_families: FontStore,
     compositor_api: CrossProcessCompositorApi,
     webrender_fonts: HashMap<FontIdentifier, FontKey>,
-    font_instances: HashMap<(FontKey, Au), FontInstanceKey>,
+    font_instances: HashMap<(FontKey, Au, Vec<FontVariation>), FontInstanceKey>,
     generic_fonts: ResolvedGenericFontFamilies,
 
     /// This is an optimization that allows the [`SystemFontService`] to send font data to
@@ -176,8 +177,15 @@ impl SystemFontService {
                     let _ =
                         result_sender.send(self.get_font_templates(font_descriptor, font_family));
                 },
-                SystemFontServiceMessage::GetFontInstance(identifier, pt_size, flags, result) => {
-                    let _ = result.send(self.get_font_instance(identifier, pt_size, flags));
+                SystemFontServiceMessage::GetFontInstance(
+                    identifier,
+                    pt_size,
+                    flags,
+                    variations,
+                    result,
+                ) => {
+                    let _ =
+                        result.send(self.get_font_instance(identifier, pt_size, flags, variations));
                 },
                 SystemFontServiceMessage::GetFontKey(result_sender) => {
                     self.fetch_new_keys();
@@ -281,6 +289,7 @@ impl SystemFontService {
         identifier: FontIdentifier,
         pt_size: Au,
         flags: FontInstanceFlags,
+        variations: Vec<FontVariation>,
     ) -> FontInstanceKey {
         self.fetch_new_keys();
 
@@ -301,7 +310,7 @@ impl SystemFontService {
 
         *self
             .font_instances
-            .entry((font_key, pt_size))
+            .entry((font_key, pt_size, variations.clone()))
             .or_insert_with(|| {
                 let font_instance_key = self.free_font_instance_keys.pop().unwrap();
                 compositor_api.add_font_instance(
@@ -309,6 +318,7 @@ impl SystemFontService {
                     font_key,
                     pt_size.to_f32_px(),
                     flags,
+                    variations,
                 );
                 font_instance_key
             })
@@ -473,6 +483,7 @@ impl SystemFontServiceProxy {
         identifier: FontIdentifier,
         size: Au,
         flags: FontInstanceFlags,
+        variations: Vec<FontVariation>,
     ) -> FontInstanceKey {
         let (response_chan, response_port) = ipc::channel().expect("failed to create IPC channel");
         self.sender
@@ -481,6 +492,7 @@ impl SystemFontServiceProxy {
                 identifier,
                 size,
                 flags,
+                variations,
                 response_chan,
             ))
             .expect("failed to send message to system font service");