perf(sidecar): avoid second hardware warmup readback

crates/ely_servo_host/tests/hardware_rendering_context.rs

@@ -14,6 +14,7 @@
 
 use dpi::PhysicalSize;
 use ely_servo_host::HardwareOffscreenContext;
+use servo::RenderingContext;
 
 #[test]
 fn constructs_or_explains_why_not() {
@@ -52,6 +53,8 @@ fn extracts_iosurface_mach_port_from_current_surface() -> Result<(), String> {
         }
     };
 
+    context.prepare_for_rendering();
+    context.present();
     let first = context
         .current_iosurface_mach_port()
         .map_err(|error| format!("first IOSurface mach port extraction failed: {error:?}"))?;

crates/ely_servo_host/tests/live_perf_bench.rs

@@ -163,6 +163,10 @@ fn run_live_bench() -> Result<(), Box<dyn Error>> {
     );
     if kind == "hardware" {
         let full_readback_budget = viewport_bytes * u64::from(frames);
+        assert_eq!(
+            outcome.readback_rgba_bytes, viewport_bytes,
+            "hardware path should read back only the initial visible frame"
+        );
         assert!(
             total_rgba_bytes < full_readback_budget,
             "hardware path stayed on full readback: {total_rgba_bytes} >= {full_readback_budget}"