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/zeta/src/completion_diff_element.rs

@@ -2,8 +2,8 @@ use std::cmp;
 
 use crate::InlineCompletion;
 use gpui::{
-    point, prelude::*, quad, size, AnyElement, App, Bounds, Corners, Edges, HighlightStyle, Hsla,
-    StyledText, TextLayout, TextStyle,
+    point, prelude::*, quad, size, AnyElement, App, BorderStyle, Bounds, Corners, Edges,
+    HighlightStyle, Hsla, StyledText, TextLayout, TextStyle,
 };
 use language::OffsetRangeExt;
 use settings::Settings;
@@ -150,6 +150,7 @@ impl Element for CompletionDiffElement {
                 cx.theme().colors().editor_active_line_background,
                 Edges::default(),
                 Hsla::transparent_black(),
+                BorderStyle::default(),
             ));
             self.element.paint(window, cx);
             window.paint_quad(quad(
@@ -158,6 +159,7 @@ impl Element for CompletionDiffElement {
                 cx.theme().players().local().cursor,
                 Edges::default(),
                 Hsla::transparent_black(),
+                BorderStyle::default(),
             ));
         }
     }