Auto merge of #18222 - canaltinova:font-feature-values-lookup, r=emilo,xidorn

stylo: Implement font feature values lookup

This PR reviewed by emilio and xidorn on bugzilla

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes [Bug 1355721](https://bugzilla.mozilla.org/show_bug.cgi?id=1355721)

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ports/geckolib/glue.rs

@@ -88,6 +88,7 @@ use style::gecko_bindings::structs::ServoElementSnapshotTable;
 use style::gecko_bindings::structs::ServoTraversalFlags;
 use style::gecko_bindings::structs::StyleRuleInclusion;
 use style::gecko_bindings::structs::URLExtraData;
+use style::gecko_bindings::structs::gfxFontFeatureValueSet;
 use style::gecko_bindings::structs::nsCSSValueSharedList;
 use style::gecko_bindings::structs::nsCompatibility;
 use style::gecko_bindings::structs::nsIDocument;
@@ -3519,6 +3520,30 @@ pub extern "C" fn Servo_StyleSet_GetCounterStyleRule(raw_data: RawServoStyleSetB
     }).unwrap_or(ptr::null_mut())
 }
 
+#[no_mangle]
+pub extern "C" fn Servo_StyleSet_BuildFontFeatureValueSet(
+  raw_data: RawServoStyleSetBorrowed,
+  set: *mut gfxFontFeatureValueSet
+) -> bool {
+    let data = PerDocumentStyleData::from_ffi(raw_data).borrow();
+
+    let global_style_data = &*GLOBAL_STYLE_DATA;
+    let guard = global_style_data.shared_lock.read();
+
+    let font_feature_values_iter = data.extra_style_data
+        .iter_origins_rev()
+        .flat_map(|(d, _)| d.font_feature_values.iter());
+
+    let mut any_rule = false;
+    for src in font_feature_values_iter {
+        any_rule = true;
+        let rule = src.read_with(&guard);
+        rule.set_at_rules(set);
+    }
+
+    any_rule
+}
+
 #[no_mangle]
 pub extern "C" fn Servo_StyleSet_ResolveForDeclarations(
     raw_data: RawServoStyleSetBorrowed,