Extend border radius on relevant widgets

core/src/border_radius.rs

@@ -0,0 +1,22 @@
+/// 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 BorderRadius([f32; 4]);
+
+impl From<f32> for BorderRadius {
+    fn from(w: f32) -> Self {
+        Self([w; 4])
+    }
+}
+
+impl From<[f32; 4]> for BorderRadius {
+    fn from(radi: [f32; 4]) -> Self {
+        Self(radi)
+    }
+}
+
+impl From<BorderRadius> for [f32; 4] {
+    fn from(radi: BorderRadius) -> Self {
+        radi.0
+    }
+}