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/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")
+                },
             }
         }
     }