compositing: Move image output and shutdown management out of the compositor (#35538)

This is a step toward the renderer-per-WebView goal. It moves various
details out of `IOCompositor`.

- Image output: This is moved to servoshell as now applications can
  access the image contents of a `WebView` via
  `RenderingContext::read_to_image`. Most options for this are moved to
  `ServoShellPreferences` apart from `wait_for_stable_image` as this
  requires a specific kind of coordination in the `ScriptThread` that is
  also very expensive. Instead, paint is now simply delayed until a
  stable image is reached and `WebView::paint()` returns a boolean.
  Maybe this can be revisited in the future.
- Shutdown: Shutdown is now managed by libservo itself. Shutdown state
  is shared between the compositor and `Servo` instance. In the future,
  this sharing might be unecessary.
- `CompositeTarget` has been removed entirely. This no longer needs to
   be passed when creating a Servo instance.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Ngo Iok Ui (Wu Yu Wei) <yuweiwu@pm.me>

ports/servoshell/desktop/cli.rs

@@ -29,7 +29,8 @@ pub fn main() {
     crate::init_tracing(servoshell_preferences.tracing_filter.as_deref());
 
     let clean_shutdown = servoshell_preferences.clean_shutdown;
-    let event_loop = EventsLoop::new(servoshell_preferences.headless, opts.output_file.is_some())
+    let has_output_file = servoshell_preferences.output_image_path.is_some();
+    let event_loop = EventsLoop::new(servoshell_preferences.headless, has_output_file)
         .expect("Failed to create events loop");
 
     {