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/dom/window.rs

@@ -65,7 +65,7 @@ use script_traits::{
 use selectors::attr::CaseSensitivity;
 use servo_arc::Arc as ServoArc;
 use servo_atoms::Atom;
-use servo_config::pref;
+use servo_config::{opts, pref};
 use servo_geometry::{f32_rect_to_au_rect, DeviceIndependentIntRect, MaxRect};
 use servo_url::{ImmutableOrigin, MutableOrigin, ServoUrl};
 use style::dom::OpaqueNode;
@@ -368,9 +368,6 @@ pub(crate) struct Window {
     /// 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 Css.
     unminify_css: bool,
 
@@ -2071,7 +2068,7 @@ impl Window {
         // When all these conditions are met, notify the constellation
         // that this pipeline is ready to write the image (from the script thread
         // perspective at least).
-        if self.prepare_for_screenshot && updating_the_rendering {
+        if opts::get().wait_for_stable_image && updating_the_rendering {
             // Checks if the html element has reftest-wait attribute present.
             // See http://testthewebforward.org/docs/reftests.html
             // and https://web-platform-tests.org/writing-tests/crashtest.html
@@ -2166,7 +2163,7 @@ impl Window {
     /// If writing a screenshot, synchronously update the layout epoch that it set
     /// in the constellation.
     pub(crate) fn update_constellation_epoch(&self) {
-        if !self.prepare_for_screenshot {
+        if !opts::get().wait_for_stable_image {
             return;
         }
 
@@ -2772,7 +2769,6 @@ impl Window {
         webrender_document: DocumentId,
         compositor_api: CrossProcessCompositorApi,
         relayout_event: bool,
-        prepare_for_screenshot: bool,
         unminify_js: bool,
         unminify_css: bool,
         local_script_source: Option<String>,
@@ -2862,7 +2858,6 @@ impl Window {
             compositor_api,
             has_sent_idle_message: Cell::new(false),
             relayout_event,
-            prepare_for_screenshot,
             unminify_css,
             userscripts_path,
             player_context,