script: do not update Document rendering when waiting on asynchronous canvas image updates (#39022)

This is fixup for #37776. We forget to skip documents with
waiting_on_canvas_image_updates flag.

Testing: Existing WPT tests and manual testing
Fixes: #39021

---------

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>

components/script/script_thread.rs

@@ -1158,6 +1158,10 @@ impl ScriptThread {
                 continue;
             }
 
+            if document.waiting_on_canvas_image_updates() {
+                continue;
+            }
+
             // TODO(#31581): The steps in the "Revealing the document" section need to be implemented
             // `process_pending_input_events` handles the focusing steps as well as other events
             // from the compositor.