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/webgpu/lib.rs

@@ -2,9 +2,9 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+use canvas_context::WGPUImageMap;
+pub use canvas_context::{ContextData, WGPUExternalImages};
 use log::warn;
-use swapchain::WGPUImageMap;
-pub use swapchain::{ContextData, WGPUExternalImages};
 use webgpu_traits::{WebGPU, WebGPUMsg};
 use wgpu_thread::WGPU;
 pub use {wgpu_core as wgc, wgpu_types as wgt};
@@ -19,7 +19,7 @@ use compositing_traits::{CrossProcessCompositorApi, WebrenderExternalImageRegist
 use ipc_channel::ipc::{self, IpcReceiver};
 use servo_config::pref;
 
-pub mod swapchain;
+pub mod canvas_context;
 
 pub fn start_webgpu_thread(
     compositor_api: CrossProcessCompositorApi,