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/gpui/src/scene.rs

@@ -455,7 +455,7 @@ pub(crate) enum PrimitiveBatch<'a> {
 #[repr(C)]
 pub(crate) struct Quad {
     pub order: DrawOrder,
-    pub pad: u32, // align to 8 bytes
+    pub border_style: BorderStyle,
     pub bounds: Bounds<ScaledPixels>,
     pub content_mask: ContentMask<ScaledPixels>,
     pub background: Background,
@@ -505,6 +505,17 @@ impl From<Shadow> for Primitive {
     }
 }
 
+/// The style of a border.
+#[derive(Default, Debug, Clone, Copy, PartialEq, Eq, Hash)]
+#[repr(C)]
+pub enum BorderStyle {
+    /// A solid border.
+    #[default]
+    Solid = 0,
+    /// A dashed border.
+    Dashed = 1,
+}
+
 /// A data type representing a 2 dimensional transformation that can be applied to an element.
 #[derive(Debug, Clone, Copy, PartialEq)]
 #[repr(C)]