compositor: Make input event handling per-WebView (#35716)

This is another step in the move to having a per-WebView renderer. In
this step event handling is made per-WebView. Most events sent to Servo
are sent via the WebView API already, so this just moves more event
handling code to the per-WebView render portion of the compositor.

- ServoRenderer is given shared ownership and interior mutability as
  it is now shared among all WebView(Renderers).
- Some messages coming from other parts of Servo must now carry a
  WebViewId as well so that they can be associated with a particular
  WebView.
- There needs to be some reorganization of `ServoRenderer` in order to
  avoid issues with double borrow of `RefCells`.

Signed-off-by: Delan Azabani <dazabani@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>

components/constellation/tracing.rs

@@ -82,7 +82,7 @@ mod from_compositor {
                 Self::SendError(..) => target!("SendError"),
                 Self::FocusWebView(..) => target!("FocusWebView"),
                 Self::BlurWebView => target!("BlurWebView"),
-                Self::ForwardInputEvent(event, ..) => event.log_target(),
+                Self::ForwardInputEvent(_webview_id, event, ..) => event.log_target(),
                 Self::SetCursor(..) => target!("SetCursor"),
                 Self::ToggleProfiler(..) => target!("EnableProfiler"),
                 Self::ExitFullScreen(_) => target!("ExitFullScreen"),