Move Servo IPC off UI thread

Root cause of the post-tab lag: the GPUI 16 ms timer was calling
`WebSurfaceRuntime::ensure_tab` and `tick` on the UI thread, and each
call did a synchronous `serde_json` write plus `read_line` against the
Servo sidecar over stdin/stdout. With even one visible tab, every
frame stalled on cross-process IPC.

Introduce `web_surface_worker.rs` — a per-profile worker thread that
owns the `ServoLiveClient`, drains a coalescing request queue
(latest Ensure/Poll per tab wins, no unbounded growth), and ships
results back through a `std::sync::mpsc` channel. `WebSurfaceRuntime`
now submits work non-blockingly and drains responses in `tick`; the
UI thread never blocks on the sidecar.

Adjacent in-flight cleanup riding along: hardware IOSurface
rendering-context completion (sidecar `live_protocol`,
`hardware_rendering_context`, GPUI BGRA surface shader), CSS viewport
size + device pixel ratio plumbing into `ServoLiveFrame`, and the
Send opt-ins for `CVPixelBuffer`-bearing types so frames can cross
the thread boundary.

third_party/gpui/src/scene.rs

@@ -658,6 +658,7 @@ pub(crate) struct PaintSurface {
     pub order: DrawOrder,
     pub bounds: Bounds<ScaledPixels>,
     pub content_mask: ContentMask<ScaledPixels>,
+    pub corner_radii: Corners<ScaledPixels>,
     #[cfg(target_os = "macos")]
     pub image_buffer: core_video::pixel_buffer::CVPixelBuffer,
 }