libservo: Expose an OffscreenRenderingContext and use it for servoshell (#35465)

Create a new `RenderingContext` which is used to render to a
`SurfmanRenderingContext`-related offscreen buffer. This allows having a
temporary place to render Servo and then blitting the results to a
subsection of the parent `RenderingContext`.

The goal with this change is to remove the details of how servoshell
renders from the `Compositor` and prepare for the compositor-per-WebView
world.


Co-authred-by: Ngo Iok Ui (Wu Yu Wei) <yuweiwu@pm.me>

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Mukilan Thiyagarajan <mukilan@igalia.com>

python/servo/testing_commands.py

@@ -161,7 +161,6 @@ class MachCommands(CommandBase):
         self_contained_tests = [
             "background_hang_monitor",
             "base",
-            "compositing",
             "constellation",
             "fonts",
             "hyper_serde",
@@ -175,6 +174,7 @@ class MachCommands(CommandBase):
             "servo_config",
             "servoshell",
             "style_config",
+            "webrender_traits",
         ]
         if not packages:
             packages = set(os.listdir(path.join(self.context.topdir, "tests", "unit"))) - set(['.DS_Store'])