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/webgpu/messages/recv.rs

@@ -12,6 +12,7 @@ use ipc_channel::ipc::{IpcSender, IpcSharedMemory};
 use pixels::IpcSnapshot;
 use serde::{Deserialize, Serialize};
 use webrender_api::ImageKey;
+use webrender_api::euclid::default::Size2D;
 use webrender_api::units::DeviceIntSize;
 use wgpu_core::Label;
 use wgpu_core::binding_model::{
@@ -52,6 +53,13 @@ use crate::{
     WebGPURenderPipelineResponse,
 };
 
+#[derive(Debug, Deserialize, Serialize)]
+pub struct PendingTexture {
+    pub texture_id: TextureId,
+    pub encoder_id: CommandEncoderId,
+    pub configuration: ContextConfiguration,
+}
+
 #[derive(Debug, Deserialize, Serialize)]
 pub enum WebGPURequest {
     BufferMapAsync {
@@ -152,22 +160,18 @@ pub enum WebGPURequest {
         size: DeviceIntSize,
         sender: IpcSender<(WebGPUContextId, ImageKey)>,
     },
-    /// Recreates swapchain (if needed)
-    UpdateContext {
+    /// Present texture to WebRender
+    Present {
         context_id: WebGPUContextId,
-        size: DeviceIntSize,
-        configuration: Option<ContextConfiguration>,
+        pending_texture: Option<PendingTexture>,
+        size: Size2D<u32>,
+        canvas_epoch: Epoch,
     },
-    /// Reads texture to swapchains buffer and maps it
-    SwapChainPresent {
-        context_id: WebGPUContextId,
-        texture_id: TextureId,
-        encoder_id: CommandEncoderId,
-        canvas_epoch: Option<Epoch>,
-    },
-    /// Obtains image from latest presentation buffer (same as wr update)
+    /// Create [`pixels::Snapshot`] with contents of the last present operation
+    /// or provided pending texture and send it over provided [`IpcSender`].
     GetImage {
         context_id: WebGPUContextId,
+        pending_texture: Option<PendingTexture>,
         sender: IpcSender<IpcSnapshot>,
     },
     ValidateTextureDescriptor {