compositor: Allow canvas to upload rendered contents asynchronously (#37776)

Adds epoch to each WR image op command that is sent to compositor. The
renderer now has a `FrameDelayer` data structure that is responsible for
tracking when a frame is ready to be displayed. When asking canvases to
update their rendering, they are given an optional `Epoch` which denotes
the `Document`'s canvas epoch. When all image updates for that `Epoch`
are seen in the renderer, the frame can be displayed.

Testing: Existing WPT tests
Fixes: #35733

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>

components/constellation/constellation.rs

@@ -1458,6 +1458,9 @@ where
             EmbedderToConstellationMessage::TickAnimation(webview_ids) => {
                 self.handle_tick_animation(webview_ids)
             },
+            EmbedderToConstellationMessage::NoLongerWaitingOnAsynchronousImageUpdates(
+                pipeline_ids,
+            ) => self.handle_no_longer_waiting_on_asynchronous_image_updates(pipeline_ids),
             EmbedderToConstellationMessage::WebDriverCommand(command) => {
                 self.handle_webdriver_msg(command);
             },
@@ -3498,6 +3501,20 @@ where
         }
     }
 
+    #[servo_tracing::instrument(skip_all)]
+    fn handle_no_longer_waiting_on_asynchronous_image_updates(
+        &mut self,
+        pipeline_ids: Vec<PipelineId>,
+    ) {
+        for pipeline_id in pipeline_ids.into_iter() {
+            if let Some(pipeline) = self.pipelines.get(&pipeline_id) {
+                let _ = pipeline.event_loop.send(
+                    ScriptThreadMessage::NoLongerWaitingOnAsychronousImageUpdates(pipeline_id),
+                );
+            }
+        }
+    }
+
     /// Schedule a navigation(via load_url).
     /// 1: Ask the embedder for permission.
     /// 2: Store the details of the navigation, pending approval from the embedder.

components/constellation/tracing.rs

@@ -79,6 +79,9 @@ mod from_compositor {
                 Self::SendImageKeysForPipeline(..) => target!("SendImageKeysForPipeline"),
                 Self::SetWebDriverResponseSender(..) => target!("SetWebDriverResponseSender"),
                 Self::PreferencesUpdated(..) => target!("PreferencesUpdated"),
+                Self::NoLongerWaitingOnAsynchronousImageUpdates(..) => {
+                    target!("NoLongerWaitingOnCanvas")
+                },
             }
         }
     }