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/headed_window.rs

@@ -17,7 +17,6 @@ use raw_window_handle::{HasDisplayHandle, HasWindowHandle};
 use servo::compositing::windowing::{
     AnimationState, EmbedderCoordinates, WebRenderDebugOption, WindowMethods,
 };
-use servo::config::opts::Opts;
 use servo::servo_config::pref;
 use servo::servo_geometry::DeviceIndependentPixel;
 use servo::webrender_api::units::{DeviceIntPoint, DeviceIntRect, DeviceIntSize, DevicePixel};
@@ -78,24 +77,17 @@ pub struct Window {
 
 impl Window {
     pub fn new(
-        opts: &Opts,
         servoshell_preferences: &ServoShellPreferences,
         event_loop: &ActiveEventLoop,
     ) -> Window {
-        // If there's no chrome, start off with the window invisible. It will be set to visible in
-        // `load_end()`. This avoids an ugly flash of unstyled content (especially important since
-        // unstyled content is white and chrome often has a transparent background). See issue
-        // #9996.
         let no_native_titlebar = servoshell_preferences.no_native_titlebar;
-        let visible = opts.output_file.is_none() && !servoshell_preferences.no_native_titlebar;
-
         let window_size = servoshell_preferences.initial_window_size;
         let window_attr = winit::window::Window::default_attributes()
             .with_title("Servo".to_string())
             .with_decorations(!no_native_titlebar)
             .with_transparent(no_native_titlebar)
             .with_inner_size(LogicalSize::new(window_size.width, window_size.height))
-            .with_visible(visible);
+            .with_visible(true);
 
         #[allow(deprecated)]
         let winit_window = event_loop