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>

tests/wpt/meta/MANIFEST.json

@@ -506147,7 +506147,7 @@
         []
        ],
        "executorservo.py": [
-        "e3369d24ebc6d6aac1b4632ef673a36602417745",
+        "2710b1b844c8e93c251c91c8809697d357bd4f06",
         []
        ],
        "executorservodriver.py": [

tests/wpt/tests/tools/wptrunner/wptrunner/executors/executorservo.py

@@ -226,6 +226,7 @@ class ServoRefTestExecutor(ServoExecutor):
 
     def screenshot(self, test, viewport_size, dpi, page_ranges):
         with TempFilename(self.tempdir) as output_path:
+            output_path = f"{output_path}.png"
             extra_args = ["--exit",
                           "--output=%s" % output_path,
                           "--window-size", viewport_size or "800x600"]