Bug 1963212 - Reuse FontFaceImpl more aggressively. r=jfkthame

When deep-cloning a stylesheet (like we do for devtools), we end up with
new (identical) LockedFontFaceRule object, but we'd fail to re-use it,
triggering new font loads. Since we don't cache failed loads, this
caused flashing and a temporary state where the used fonts were not what
this DevTools test expects.

Rejigger the code a little bit to push down the decision of whether to
reuse the FontFaceImpl object further down. That allows us to reuse the
FontFace object for the stylesheet caching / deep cloning use-case.

Differential Revision: https://phabricator.services.mozilla.com/D247704

servo/ports/geckolib/glue.rs

@@ -3251,6 +3251,21 @@ pub unsafe extern "C" fn Servo_FontFaceRule_Clone(
     with_maybe_worker_shared_lock(|lock| Arc::new(lock.wrap(clone)).into())
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn Servo_FontFaceRule_Equals(
+    a: &LockedFontFaceRule,
+    b: &LockedFontFaceRule,
+) -> bool {
+    if a as *const _ == b as *const _ {
+        return true;
+    }
+    read_locked_arc_worker(a, |a: &FontFaceRule| {
+        read_locked_arc_worker(b, |b: &FontFaceRule| {
+            a == b
+        })
+    })
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn Servo_FontFaceRule_GetSourceLocation(
     rule: &LockedFontFaceRule,