Add snapshot test for image blending (#7309)

This adds a test that can be used to see the improvements made by this
PR (if any):
* https://github.com/emilk/egui/pull/5839

crates/egui_demo_lib/tests/image_blending.rs

@@ -0,0 +1,25 @@
+use egui::{hex_color, include_image};
+use egui_kittest::Harness;
+
+#[test]
+fn test_image_blending() {
+    for pixels_per_point in [1.0, 2.0] {
+        let mut harness = Harness::builder()
+            .with_pixels_per_point(pixels_per_point)
+            .build_ui(|ui| {
+                egui_extras::install_image_loaders(ui.ctx());
+                egui::Frame::new()
+                    .fill(hex_color!("#5981FF"))
+                    .show(ui, |ui| {
+                        ui.add(
+                            egui::Image::new(include_image!("../data/ring.png"))
+                                .max_height(18.0)
+                                .tint(egui::Color32::GRAY),
+                        );
+                    });
+            });
+        harness.run();
+        harness.fit_contents();
+        harness.snapshot(format!("image_blending/image_x{pixels_per_point}"));
+    }
+}