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/egl/android/simpleservo.rs

@@ -7,7 +7,6 @@ use std::mem;
 use std::rc::Rc;
 
 use raw_window_handle::{DisplayHandle, RawDisplayHandle, RawWindowHandle, WindowHandle};
-use servo::compositing::CompositeTarget;
 pub use servo::webrender_api::units::DeviceIntRect;
 /// The EventLoopWaker::wake function will be called from any thread.
 /// It will be called to notify embedder that some events are available,
@@ -97,7 +96,6 @@ pub fn init(
         embedder_callbacks,
         window_callbacks.clone(),
         None,
-        CompositeTarget::ContextFbo,
     );
 
     APP.with(|app| {