Paint mouse cursor in kittest snapshot images (#7721)

Very simple triangle shape, but helps understand why a widget has a
hovered effect

Author: emilk

crates/egui_demo_app/tests/snapshots/clock.png

@@ -657,7 +657,28 @@ impl<'a, State> Harness<'a, State> {
     /// Returns an error if the rendering fails.
     #[cfg(any(feature = "wgpu", feature = "snapshot"))]
     pub fn render(&mut self) -> Result<image::RgbaImage, String> {
-        self.renderer.render(&self.ctx, &self.output)
+        let mut output = self.output.clone();
+
+        if let Some(mouse_pos) = self.ctx.input(|i| i.pointer.hover_pos()) {
+            // Paint a mouse cursor:
+            let triangle = vec![
+                mouse_pos,
+                mouse_pos + egui::vec2(16.0, 8.0),
+                mouse_pos + egui::vec2(8.0, 16.0),
+            ];
+
+            output.shapes.push(ClippedShape {
+                clip_rect: self.ctx.content_rect(),
+                shape: egui::epaint::PathShape::convex_polygon(
+                    triangle,
+                    Color32::WHITE,
+                    egui::Stroke::new(1.0, Color32::BLACK),
+                )
+                .into(),
+            });
+        }
+
+        self.renderer.render(&self.ctx, &output)
     }
 
     /// Get the root viewport output