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/compositing/lib.rs

@@ -4,16 +4,18 @@
 
 #![deny(unsafe_code)]
 
+use std::cell::Cell;
 use std::rc::Rc;
 
 use compositing_traits::{CompositorProxy, CompositorReceiver, ConstellationMsg};
 use crossbeam_channel::Sender;
+use embedder_traits::ShutdownState;
 use profile_traits::{mem, time};
 use webrender::RenderApi;
 use webrender_api::DocumentId;
 use webrender_traits::rendering_context::RenderingContext;
 
-pub use crate::compositor::{CompositeTarget, IOCompositor, ShutdownState};
+pub use crate::compositor::IOCompositor;
 
 #[macro_use]
 mod tracing;
@@ -35,6 +37,9 @@ pub struct InitialCompositorState {
     pub time_profiler_chan: time::ProfilerChan,
     /// A channel to the memory profiler thread.
     pub mem_profiler_chan: mem::ProfilerChan,
+    /// A shared state which tracks whether Servo has started or has finished
+    /// shutting down.
+    pub shutdown_state: Rc<Cell<ShutdownState>>,
     /// Instance of webrender API
     pub webrender: webrender::Renderer,
     pub webrender_document: DocumentId,