script: Do not iterate through all image frames when advancing animated images (#38857)

Instead of iterating through all frames every time we are looking for a
particular frame, just index into the `Vec` of frames directly. This
should do less work on every frame transition.

Testing: This is just a small optimization, so shouldn't change
observable
behavior.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/image_animation.rs

@@ -72,8 +72,7 @@ impl ImageAnimationManager {
 
                 let image = &state.image;
                 let frame = image
-                    .frames()
-                    .nth(state.active_frame)
+                    .frame(state.active_frame)
                     .expect("active_frame should within range of frames");
 
                 if let Some(node) = rooted_nodes.get(&NoTrace(*node)) {