Simplify pick_rounded_rect (#15065)

# Objective

Simplify `pick_rounded_rect` with multiple `if` statements to make it
more readable and efficient([Godbolt
link](https://godbolt.org/z/W5vPEvT5c)).

Co-authored-by: WX\shixi <shixi1@cnwxsoft.com>

crates/bevy_ui/src/focus.rs

@@ -352,12 +352,12 @@ pub(crate) fn pick_rounded_rect(
  size: Vec2,
  border_radius: ResolvedBorderRadius,
 ) -> bool {
- let s = point.signum();
- let r = (border_radius.top_left * (1. - s.x) * (1. - s.y)
-  + border_radius.top_right * (1. + s.x) * (1. - s.y)
- + border_radius.bottom_right * (1. + s.x) * (1. + s.y)
-  + border_radius.bottom_left * (1. - s.x) * (1. + s.y))
-  / 4.;
+ let [top, bottom] = if point.x < 0. {
+  [border_radius.top_left, border_radius.bottom_left]
+ } else {
+ [border_radius.top_right, border_radius.bottom_right]
+ };
+ let r = if point.y < 0. { top } else { bottom };
 
  let corner_to_point = point.abs() - 0.5 * size;
  let q = corner_to_point + r;