test(paint): pin the retouch dab footprint to the Full Size ring (#1761) (#3258)

A dab never paints past radius + 0.5 px from its centre, at any hardness:
the half pixel is the anti-aliased edge of dab_coverage. A hard tip does
reach the ring; softer tips fade out inside it, which is the falloff
working as intended.

crates/paint/src/retouch.rs

@@ -423,6 +423,7 @@ fn premultiply(px: &mut [f32], alpha: Option<usize>) {
 mod tests {
     use super::*;
     use crate::{BrushSettings, StrokePoint};
+    use photocraft_geom::Point;
 
     fn stroke(points: &[(f64, f64)], size: f32, hardness: f32) -> Stroke {
         Stroke {
@@ -551,4 +552,39 @@ mod tests {
         mix_premultiplied(&mut d, &[1.0, 1.0], 7, 0.5);
         assert_eq!(d, [0.5, 0.5]);
     }
+
+    #[test]
+    fn footprint_stays_inside_the_full_size_ring() {
+        let radius = 10.5f32;
+        for hardness in [1.0, 0.5, 0.0] {
+            let dab = Dab::round(Point::new(32.0, 32.0), radius, 1.0);
+            let fp = footprint(&dab, 0, hardness);
+            let mut painted = 0usize;
+            let mut farthest = 0.0f64;
+            for y in fp.rect.y0..fp.rect.y1 {
+                for x in fp.rect.x0..fp.rect.x1 {
+                    let dist = ((x as f64 + 0.5 - dab.center.x).powi(2) + (y as f64 + 0.5 - dab.center.y).powi(2)).sqrt();
+                    if fp.at(x, y) <= 0.0 {
+                        continue;
+                    }
+                    painted += 1;
+                    farthest = farthest.max(dist);
+                    assert!(
+                        dist < radius as f64 + 0.5,
+                        "hardness {hardness}: pixel ({x}, {y}) at {dist} px from \
+                         the centre is painted past the Full Size ring of \
+                         {radius} px"
+                    );
+                }
+            }
+            assert!(painted > 0, "hardness {hardness}: nothing painted");
+            if hardness == 1.0 {
+                assert!(
+                    farthest > radius as f64,
+                    "a hard tip painted only {farthest} px out, so the Full \
+                     Size ring at {radius} px is never reached"
+                );
+            }
+        }
+    }
 }