compositing: Support per-Painter operations and message handling (#40788)

Currently, only the first `Painter` is used for all operations in the
`Compositor`. This change modifies the compositor API and message
handling to allow routing the operations to the correct `Painter` via a
provided `PainterId` or `WebViewId`.

This change is to enable support for per-`WebView`s `RenderingContext`s.

Testing: This change shouldn't change behavior, so existing WPT tests
should cover it.

---------

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/document.rs

@@ -2799,6 +2799,7 @@ impl Document {
             results.insert(ReflowPhasesRun::UpdatedImageData);
             self.waiting_on_canvas_image_updates.set(true);
             self.window().compositor_api().delay_new_frame_for_canvas(
+                self.webview_id(),
                 self.window().pipeline_id(),
                 current_rendering_epoch,
                 image_keys,