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>

components/script/script_thread.rs

@@ -298,9 +298,6 @@ pub struct ScriptThread {
     /// Emits notifications when there is a relayout.
     relayout_event: bool,
 
-    /// True if it is safe to write to the image.
-    prepare_for_screenshot: bool,
-
     /// Unminify Javascript.
     unminify_js: bool,
 
@@ -835,10 +832,6 @@ impl ScriptThread {
         system_font_service: Arc<SystemFontServiceProxy>,
         user_agent: Cow<'static, str>,
     ) -> ScriptThread {
-        let opts = opts::get();
-        let prepare_for_screenshot =
-            opts.output_file.is_some() || opts.exit_after_load || opts.webdriver_port.is_some();
-
         let (self_sender, self_receiver) = unbounded();
         let runtime = Runtime::new(Some(SendableTaskSource {
             sender: ScriptEventLoopSender::MainThread(self_sender.clone()),
@@ -898,6 +891,7 @@ impl ScriptThread {
             webgpu_receiver: RefCell::new(crossbeam_channel::never()),
         };
 
+        let opts = opts::get();
         let senders = ScriptThreadSenders {
             self_sender,
             #[cfg(feature = "bluetooth")]
@@ -946,7 +940,6 @@ impl ScriptThread {
             profile_script_events: opts.debug.profile_script_events,
             print_pwm: opts.print_pwm,
             relayout_event: opts.debug.relayout_event,
-            prepare_for_screenshot,
             unminify_js: opts.unminify_js,
             local_script_source: opts.local_script_source.clone(),
             unminify_css: opts.unminify_css,
@@ -3099,7 +3092,6 @@ impl ScriptThread {
             self.webrender_document,
             self.compositor_api.clone(),
             self.relayout_event,
-            self.prepare_for_screenshot,
             self.unminify_js,
             self.unminify_css,
             self.local_script_source.clone(),