Add improved pixel alignment test with alternating white/black lines (#4537)

This is a good test, because you will have obvious darkening/lightening
when this fails, and/or Moiré patterns.

<img width="490" alt="image"
src="https://github.com/emilk/egui/assets/1148717/bee74b9f-2529-4544-9102-2ee273eade2d">

crates/egui_demo_lib/src/rendering_test.rs

@@ -398,12 +398,27 @@ impl TextureManager {
 /// Requires eyes and a magnifying glass to verify.
 pub fn pixel_test(ui: &mut Ui) {
     ui.heading("Pixel alignment test");
+    ui.label("If anything is blurry, then everything will be blurry, including text.");
+    ui.label("You might need a magnifying glass to check this test.");
+
+    if cfg!(target_arch = "wasm32") {
+        ui.label("Make sure these test pass even when you zoom in/out and resize the browser.");
+    }
+
+    ui.add_space(4.0);
+
+    pixel_test_lines(ui);
+
+    ui.add_space(4.0);
+
+    pixel_test_squares(ui);
+}
+
+fn pixel_test_squares(ui: &mut Ui) {
     ui.label("The first square should be exactly one physical pixel big.");
     ui.label("They should be exactly one physical pixel apart.");
     ui.label("Each subsequent square should be one physical pixel larger than the previous.");
     ui.label("They should be perfectly aligned to the physical pixel grid.");
-    ui.label("If these squares are blurry, everything will be blurry, including text.");
-    ui.label("You might need a magnifying glass to check this test.");
 
     let color = if ui.style().visuals.dark_mode {
         egui::Color32::WHITE
@@ -412,6 +427,7 @@ pub fn pixel_test(ui: &mut Ui) {
     };
 
     let pixels_per_point = ui.ctx().pixels_per_point();
+
     let num_squares = (pixels_per_point * 10.0).round().max(10.0) as u32;
     let size_pixels = vec2(
         ((num_squares + 1) * (num_squares + 2) / 2) as f32,
@@ -427,17 +443,58 @@ pub fn pixel_test(ui: &mut Ui) {
     .ceil();
     for size in 1..=num_squares {
         let rect_points = Rect::from_min_size(
-            Pos2::new(
-                cursor_pixel.x / pixels_per_point,
-                cursor_pixel.y / pixels_per_point,
-            ),
-            Vec2::splat(size as f32) / pixels_per_point,
+            Pos2::new(cursor_pixel.x, cursor_pixel.y),
+            Vec2::splat(size as f32),
         );
-        painter.rect_filled(rect_points, 0.0, color);
+        painter.rect_filled(rect_points / pixels_per_point, 0.0, color);
         cursor_pixel.x += (1 + size) as f32;
     }
 }
 
+fn pixel_test_lines(ui: &mut Ui) {
+    let pixels_per_point = ui.ctx().pixels_per_point();
+    let n = (96.0 * pixels_per_point) as usize;
+
+    ui.label("The lines should be exactly one physical pixel wide, one physical pixel apart.");
+    ui.label("They should be perfectly white and black.");
+
+    let hspace_px = pixels_per_point * 4.0;
+
+    let size_px = Vec2::new(2.0 * n as f32 + hspace_px, n as f32);
+    let size_points = size_px / pixels_per_point + Vec2::splat(2.0);
+    let (response, painter) = ui.allocate_painter(size_points, Sense::hover());
+
+    let mut cursor_px = Pos2::new(
+        response.rect.min.x * pixels_per_point,
+        response.rect.min.y * pixels_per_point,
+    )
+    .ceil();
+
+    // Vertical stripes:
+    for x in 0..n / 2 {
+        let rect_px = Rect::from_min_size(
+            Pos2::new(cursor_px.x + 2.0 * x as f32, cursor_px.y),
+            Vec2::new(1.0, n as f32),
+        );
+        painter.rect_filled(rect_px / pixels_per_point, 0.0, egui::Color32::WHITE);
+        let rect_px = rect_px.translate(vec2(1.0, 0.0));
+        painter.rect_filled(rect_px / pixels_per_point, 0.0, egui::Color32::BLACK);
+    }
+
+    cursor_px.x += n as f32 + hspace_px;
+
+    // Horizontal stripes:
+    for y in 0..n / 2 {
+        let rect_px = Rect::from_min_size(
+            Pos2::new(cursor_px.x, cursor_px.y + 2.0 * y as f32),
+            Vec2::new(n as f32, 1.0),
+        );
+        painter.rect_filled(rect_px / pixels_per_point, 0.0, egui::Color32::WHITE);
+        let rect_px = rect_px.translate(vec2(0.0, 1.0));
+        painter.rect_filled(rect_px / pixels_per_point, 0.0, egui::Color32::BLACK);
+    }
+}
+
 fn blending_and_feathering_test(ui: &mut Ui) {
     ui.label("The left side shows how lines of different widths look.");
     ui.label("The right side tests text rendering at different opacities and sizes.");