Introduce Border struct analogous to Shadow

core/src/border.rs

@@ -0,0 +1,54 @@
+//! Draw lines around containers.
+use crate::Color;
+
+/// A border.
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub struct Border {
+    /// The color of the border.
+    pub color: Color,
+
+    /// The width of the border.
+    pub width: f32,
+
+    /// The radius of the border.
+    pub radius: Radius,
+}
+
+impl Border {
+    /// Creates a new default [`Border`] with the given [`Radius`].
+    pub fn with_radius(radius: impl Into<Radius>) -> Self {
+        Self {
+            radius: radius.into(),
+            ..Self::default()
+        }
+    }
+}
+
+/// The border radii for the corners of a graphics primitive in the order:
+/// top-left, top-right, bottom-right, bottom-left.
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub struct Radius([f32; 4]);
+
+impl From<f32> for Radius {
+    fn from(w: f32) -> Self {
+        Self([w; 4])
+    }
+}
+
+impl From<u8> for Radius {
+    fn from(w: u8) -> Self {
+        Self([f32::from(w); 4])
+    }
+}
+
+impl From<[f32; 4]> for Radius {
+    fn from(radi: [f32; 4]) -> Self {
+        Self(radi)
+    }
+}
+
+impl From<Radius> for [f32; 4] {
+    fn from(radi: Radius) -> Self {
+        radi.0
+    }
+}