Add support for dashed borders to GPUI (#27139)

Features:

* Scales dash spacing with border width.
* Laying out dashes around rounded corners.
* Varying border widths with rounded corners - now uses an ellipse for the inner edge of the border.
* When there are no rounded corners, each straight border is laid out separately, so that the dashes to meet at the corners.
* All sides of each dash are antialiased.

![image](https://github.com/user-attachments/assets/b3789a98-a5be-4f97-9736-c4e59615afe6)

![image](https://github.com/user-attachments/assets/739bdc57-4580-42c8-bfc3-6e287411a408)

Release Notes:

- N/A

---------

Co-authored-by: Michael Sloan <michael@zed.dev>
Co-authored-by: Ben <ben@zed.dev>

crates/workspace/src/pane_group.rs

@@ -803,9 +803,9 @@ mod element {
     use std::{cell::RefCell, iter, rc::Rc, sync::Arc};
 
     use gpui::{
-        px, relative, size, Along, AnyElement, App, Axis, Bounds, Element, GlobalElementId,
-        IntoElement, MouseDownEvent, MouseMoveEvent, MouseUpEvent, ParentElement, Pixels, Point,
-        Size, Style, WeakEntity, Window,
+        px, relative, size, Along, AnyElement, App, Axis, BorderStyle, Bounds, Element,
+        GlobalElementId, IntoElement, MouseDownEvent, MouseMoveEvent, MouseUpEvent, ParentElement,
+        Pixels, Point, Size, Style, WeakEntity, Window,
     };
     use gpui::{CursorStyle, Hitbox};
     use parking_lot::Mutex;
@@ -1165,6 +1165,7 @@ mod element {
                                 gpui::transparent_black(),
                                 border,
                                 cx.theme().colors().border_selected,
+                                BorderStyle::Solid,
                             ));
                         }
                     }