webgpu: Simplify presentation and handle cleared in script (#38717)

There are many important changes here:

- Generalize the presentation buffer into standalone staging buffers
that hold their own state. This allow them to be used by getImage.
- Move all clear handling to the ScriptThread and send the configuration
on each request present/getimage, thus avoiding any recreate/clearing
messages. This means that we prepare staging buffers lazily, on the
first request.

Try run for this change:
https://github.com/sagudev/servo/actions/runs/17341982368
Testing: This is covered by existing WebGPU CTS tests. There are some
bad expectations updates, but they are also on main (presumably from
last update the rendering work) although I think CTS is actually wrong
(see https://github.com/gpuweb/cts/issues/4440).
Fixes: #36820
Fixes: #37705
Fixes: #33368 (we now keep reference alive in hashmap)

---------

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>

components/shared/compositing/lib.rs

@@ -523,13 +523,9 @@ impl ExternalImageHandler for WebrenderExternalImageHandlers {
             },
             WebrenderImageHandlerType::WebGPU => {
                 let (source, size) = self.webgpu_handler.as_mut().unwrap().lock(key.0);
-                let buffer = match source {
-                    ExternalImageSource::RawData(b) => b,
-                    _ => panic!("Wrong type"),
-                };
                 ExternalImage {
                     uv: TexelRect::new(0.0, size.height as f32, size.width as f32, 0.0),
-                    source: ExternalImageSource::RawData(buffer),
+                    source,
                 }
             },
         }