Swatches: default names read a -0 component as 0

`color_name` clamps each component before rounding, but `clamp` keeps a
-0.0, so a color with a -0.0 component was named "R=-0 G=128 B=-0" (and
"K=-0", "Gray K=-0" in CMYK and Gray). Adding 0.0 after rounding turns
it into 0, as the Lab branch already does.

crates/doc/src/swatches.rs

@@ -195,9 +195,10 @@ impl Document {
 /// The default name of a solid swatch: its values in its own colour model ("C=10 M=20 Y=30 K=0",
 /// "R=255 G=128 B=0", "Gray K=40", "L=52 a=70 b=-30").
 pub fn color_name(c: Color) -> String {
-    // Clamped first, so a conversion's tiny negative never reads "-0".
-    let pct = |v: f32| (v.clamp(0.0, 1.0) * 100.0).round();
-    let byte = |v: f32| (v.clamp(0.0, 1.0) * 255.0).round();
+    // Clamped first, so a conversion's tiny negative never reads "-0". `clamp` keeps a -0.0, so
+    // `+ 0.0` turns it into 0.
+    let pct = |v: f32| (v.clamp(0.0, 1.0) * 100.0).round() + 0.0;
+    let byte = |v: f32| (v.clamp(0.0, 1.0) * 255.0).round() + 0.0;
     match c {
         Color::Cmyk { c, m, y, k } => format!("C={} M={} Y={} K={}", pct(c), pct(m), pct(y), pct(k)),
         Color::Rgb { r, g, b } => format!("R={} G={} B={}", byte(r), byte(g), byte(b)),
@@ -493,6 +494,13 @@ mod tests {
         assert_eq!(d.new_swatch_name(&Paint::solid(Color::rgb(1.0, 0.0, 0.0))), "R=255 G=0 B=0");
     }
 
+    #[test]
+    fn default_names_read_negative_zero_as_zero() {
+        assert_eq!(color_name(Color::Rgb { r: -0.0, g: 0.5, b: -0.0 }), "R=0 G=128 B=0");
+        assert_eq!(color_name(Color::Cmyk { c: -0.0, m: 0.2, y: 0.3, k: -0.0 }), "C=0 M=20 Y=30 K=0");
+        assert_eq!(color_name(Color::Gray { k: -0.0 }), "Gray K=0");
+    }
+
     #[test]
     fn free_swatch_names_match_naming_each_swatch_as_it_is_added() {
         let (mut d, _, _) = doc();