Auto merge of #17410 - asajeffrey:compositor-nested-rAF-with-reftest-wait, r=jdm

Support nested requestAnimationFrame in the presence of reftest-wait

<!-- Please describe your changes on the following line: -->

Currently, nested rAF callbacks don't get called in the presence of reftest-wait. This PR fixes that.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #17408
- [X] There are tests for these changes

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/17410)
<!-- Reviewable:end -->

components/compositing/compositor.rs

@@ -1491,19 +1491,15 @@ impl<Window: WindowMethods> IOCompositor<Window> {
         };
 
         if wait_for_stable_image {
-            match self.is_ready_to_paint_image_output() {
-                Ok(()) => {
-                    // The current image is ready to output. However, if there are animations active,
-                    // tick those instead and continue waiting for the image output to be stable AND
-                    // all active animations to complete.
-                    if self.animations_active() {
-                        self.process_animations();
-                        return Err(UnableToComposite::NotReadyToPaintImage(NotReadyToPaint::AnimationsActive));
-                    }
-                }
-                Err(result) => {
-                    return Err(UnableToComposite::NotReadyToPaintImage(result))
-                }
+            // The current image may be ready to output. However, if there are animations active,
+            // tick those instead and continue waiting for the image output to be stable AND
+            // all active animations to complete.
+            if self.animations_active() {
+                self.process_animations();
+                return Err(UnableToComposite::NotReadyToPaintImage(NotReadyToPaint::AnimationsActive));
+            }
+            if let Err(result) = self.is_ready_to_paint_image_output() {
+                return Err(UnableToComposite::NotReadyToPaintImage(result))
             }
         }