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/constellation/constellation.rs

@@ -168,7 +168,7 @@ use servo_rand::{Rng, ServoRng, SliceRandom, random};
 use servo_url::{Host, ImmutableOrigin, ServoUrl};
 use style::global_style_data::StyleThreadPool;
 #[cfg(feature = "webgpu")]
-use webgpu::swapchain::WGPUImageMap;
+use webgpu::canvas_context::WGPUImageMap;
 #[cfg(feature = "webgpu")]
 use webgpu_traits::{WebGPU, WebGPURequest};
 use webrender::RenderApiSender;