Handle ViewportCommand::Screenshot in kittest (#8256)

Makes it possible to use egui_mcp to drive apps headlessly via kittest

crates/egui_kittest/tests/screenshot.rs

@@ -0,0 +1,63 @@
+#![cfg(feature = "wgpu")]
+
+use std::sync::Arc;
+
+use egui::{Color32, ColorImage, Vec2};
+use egui_kittest::Harness;
+
+/// Requesting a screenshot via [`egui::ViewportCommand::Screenshot`] from within the app should
+/// be fulfilled by the harness (when rendering is enabled) and delivered back via
+/// [`egui::Event::Screenshot`].
+#[test]
+#[cfg(any(feature = "wgpu", feature = "snapshot"))]
+fn screenshot_viewport_command() {
+    #[derive(Default)]
+    struct State {
+        requested: bool,
+        screenshot: Option<Arc<ColorImage>>,
+    }
+
+    let mut harness = Harness::builder()
+        .with_size(Vec2::new(100.0, 80.0))
+        .build_ui_state(
+            |ui, state: &mut State| {
+                // Paint the whole content area with a known color so we can verify the capture.
+                ui.painter()
+                    .rect_filled(ui.ctx().content_rect(), 0.0, Color32::RED);
+
+                // Request a screenshot once.
+                if !state.requested {
+                    state.requested = true;
+                    ui.ctx()
+                        .send_viewport_cmd(egui::ViewportCommand::Screenshot(Default::default()));
+                }
+
+                // Capture the screenshot once it's delivered.
+                ui.input(|i| {
+                    for event in &i.raw.events {
+                        if let egui::Event::Screenshot { image, .. } = event {
+                            state.screenshot = Some(Arc::clone(image));
+                        }
+                    }
+                });
+            },
+            State::default(),
+        );
+
+    harness.run();
+
+    let screenshot = harness
+        .state()
+        .screenshot
+        .clone()
+        .expect("Expected a screenshot to be delivered via Event::Screenshot");
+
+    // The frame was filled with red, so the center pixel should be red.
+    let center = screenshot.pixels[screenshot.pixels.len() / 2];
+    assert_eq!(center, Color32::RED, "center pixel should be red");
+
+    // The screenshot should match the rendered frame size.
+    let rendered = harness.render().unwrap();
+    assert_eq!(screenshot.width() as u32, rendered.width());
+    assert_eq!(screenshot.height() as u32, rendered.height());
+}