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/shared/compositing/lib.rs

@@ -58,10 +58,6 @@ impl CompositorReceiver {
 
 /// Messages from (or via) the constellation thread to the compositor.
 pub enum CompositorMsg {
-    /// Informs the compositor that the constellation has completed shutdown.
-    /// Required because the constellation can have pending calls to make
-    /// (e.g. SetFrameTree) at the time that we send it an ExitMsg.
-    ShutdownComplete,
     /// Alerts the compositor that the given pipeline has changed whether it is running animations.
     ChangeRunningAnimationsState(PipelineId, AnimationState),
     /// Create or update a webview, given its frame tree.
@@ -118,7 +114,6 @@ pub struct CompositionPipeline {
 impl Debug for CompositorMsg {
     fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
         match *self {
-            CompositorMsg::ShutdownComplete => write!(f, "ShutdownComplete"),
             CompositorMsg::ChangeRunningAnimationsState(_, state) => {
                 write!(f, "ChangeRunningAnimationsState({:?})", state)
             },