Unify circular corner geometry and shader masks

crates/ferese-theme/src/geometry.rs

@@ -0,0 +1,19 @@
+/// Radius of a concentric inset, in the same units as the outer radius.
+pub fn inner_radius(outer: f32, padding: f32) -> f32 {
+    (outer - padding).max(0.0)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn inset_corners_preserve_the_center_and_saturate() {
+        assert_eq!(inner_radius(14.0, 4.0), 10.0);
+        assert_eq!(inner_radius(7.5, 2.25), 5.25);
+        assert_eq!(inner_radius(14.0, 0.0), 14.0);
+        assert_eq!(inner_radius(14.0, 14.0), 0.0);
+        assert_eq!(inner_radius(14.0, 20.0), 0.0);
+        assert_eq!(inner_radius(0.0, 4.0), 0.0);
+    }
+}