Make text underline and strikethrough pixel perfect crisp (#5857)

Small visual fix: pixel-align any text underline or strikethrough.
Before they could be often be blurry.

crates/epaint/src/stroke.rs

@@ -2,6 +2,8 @@
 
 use std::{fmt::Debug, sync::Arc};
 
+use emath::GuiRounding as _;
+
 use super::{emath, Color32, ColorMode, Pos2, Rect};
 
 /// Describes the width and color of a line.
@@ -34,6 +36,46 @@ impl Stroke {
     pub fn is_empty(&self) -> bool {
         self.width <= 0.0 || self.color == Color32::TRANSPARENT
     }
+
+    /// For vertical or horizontal lines:
+    /// round the stroke center to produce a sharp, pixel-aligned line.
+    pub fn round_center_to_pixel(&self, pixels_per_point: f32, coord: &mut f32) {
+        // If the stroke is an odd number of pixels wide,
+        // we want to round the center of it to the center of a pixel.
+        //
+        // If however it is an even number of pixels wide,
+        // we want to round the center to be between two pixels.
+        //
+        // We also want to treat strokes that are _almost_ odd as it it was odd,
+        // to make it symmetric. Same for strokes that are _almost_ even.
+        //
+        // For strokes less than a pixel wide we also round to the center,
+        // because it will rendered as a single row of pixels by the tessellator.
+
+        let pixel_size = 1.0 / pixels_per_point;
+
+        if self.width <= pixel_size || is_nearest_integer_odd(pixels_per_point * self.width) {
+            *coord = coord.round_to_pixel_center(pixels_per_point);
+        } else {
+            *coord = coord.round_to_pixels(pixels_per_point);
+        }
+    }
+
+    pub(crate) fn round_rect_to_pixel(&self, pixels_per_point: f32, rect: &mut Rect) {
+        // We put odd-width strokes in the center of pixels.
+        // To understand why, see `fn round_center_to_pixel`.
+
+        let pixel_size = 1.0 / pixels_per_point;
+
+        let width = self.width;
+        if width <= 0.0 {
+            *rect = rect.round_to_pixels(pixels_per_point);
+        } else if width <= pixel_size || is_nearest_integer_odd(pixels_per_point * width) {
+            *rect = rect.round_to_pixel_center(pixels_per_point);
+        } else {
+            *rect = rect.round_to_pixels(pixels_per_point);
+        }
+    }
 }
 
 impl<Color> From<(f32, Color)> for Stroke
@@ -182,3 +224,21 @@ impl From<Stroke> for PathStroke {
         }
     }
 }
+
+/// Returns true if the nearest integer is odd.
+fn is_nearest_integer_odd(x: f32) -> bool {
+    (x * 0.5 + 0.25).fract() > 0.5
+}
+
+#[test]
+fn test_is_nearest_integer_odd() {
+    assert!(is_nearest_integer_odd(0.6));
+    assert!(is_nearest_integer_odd(1.0));
+    assert!(is_nearest_integer_odd(1.4));
+    assert!(!is_nearest_integer_odd(1.6));
+    assert!(!is_nearest_integer_odd(2.0));
+    assert!(!is_nearest_integer_odd(2.4));
+    assert!(is_nearest_integer_odd(2.6));
+    assert!(is_nearest_integer_odd(3.0));
+    assert!(is_nearest_integer_odd(3.4));
+}