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/elements/surface.rs

@@ -1,6 +1,6 @@
 use crate::{
-    App, Bounds, Element, ElementId, GlobalElementId, InspectorElementId, IntoElement, LayoutId,
-    ObjectFit, Pixels, Style, StyleRefinement, Styled, Window,
+    App, Bounds, Corners, Element, ElementId, GlobalElementId, InspectorElementId, IntoElement,
+    LayoutId, ObjectFit, Pixels, Style, StyleRefinement, Styled, Window,
 };
 #[cfg(target_os = "macos")]
 use core_video::pixel_buffer::CVPixelBuffer;
@@ -25,6 +25,7 @@ impl From<CVPixelBuffer> for SurfaceSource {
 pub struct Surface {
     source: SurfaceSource,
     object_fit: ObjectFit,
+    corner_radii: Option<Corners<Pixels>>,
     style: StyleRefinement,
 }
 
@@ -33,6 +34,7 @@ pub fn surface(source: impl Into<SurfaceSource>) -> Surface {
     Surface {
         source: source.into(),
         object_fit: ObjectFit::Contain,
+        corner_radii: None,
         style: Default::default(),
     }
 }
@@ -43,6 +45,12 @@ impl Surface {
         self.object_fit = object_fit;
         self
     }
+
+    /// Set rounded clipping for the rendered surface.
+    pub fn corner_radii(mut self, corner_radii: impl Into<Corners<Pixels>>) -> Self {
+        self.corner_radii = Some(corner_radii.into());
+        self
+    }
 }
 
 impl Element for Surface {
@@ -96,8 +104,13 @@ impl Element for Surface {
             SurfaceSource::Surface(surface) => {
                 let size = crate::size(surface.get_width().into(), surface.get_height().into());
                 let new_bounds = self.object_fit.get_bounds(bounds, size);
-                // TODO: Add support for corner_radii
-                window.paint_surface(new_bounds, surface.clone());
+                let mut style = Style::default();
+                style.refine(&self.style);
+                let corner_radii = self
+                    .corner_radii
+                    .unwrap_or_else(|| style.corner_radii.to_pixels(window.rem_size()))
+                    .clamp_radii_for_quad_size(new_bounds.size);
+                window.paint_surface(new_bounds, corner_radii, surface.clone());
             }
             #[allow(unreachable_patterns)]
             _ => {}