T13: push display scale factor into Servo's hidpi so Retina pages lay out at logical CSS dimensions

crates/ely_servo_host/src/host.rs

@@ -235,6 +235,17 @@ pub struct PageZoomRequest {
     pub zoom_factor: f32,
 }
 
+/// Set the WebView's hidpi (device → CSS pixel) scale. Servo's
+/// builder defaults this to 1.0; on Retina hosts that produces
+/// half-size layout because the page treats physical pixels as CSS
+/// pixels. The embedder should mirror the platform's reported scale
+/// factor on every viewport-bound change.
+#[derive(Clone, Debug, PartialEq)]
+pub struct HidpiScaleRequest {
+    pub webview_id: WebViewId,
+    pub scale_factor: f32,
+}
+
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct MouseClickRequest {
     pub webview_id: WebViewId,
@@ -316,6 +327,8 @@ pub trait ServoHost {
 
     fn set_page_zoom(&mut self, request: PageZoomRequest) -> Result<(), ServoHostError>;
 
+    fn set_hidpi_scale(&mut self, request: HidpiScaleRequest) -> Result<(), ServoHostError>;
+
     fn click(&mut self, request: MouseClickRequest) -> Result<(), ServoHostError>;
 
     fn hover(&mut self, request: MouseHoverRequest) -> Result<(), ServoHostError>;