gpui: Add linear gradient support to fill background (#20812)

Release Notes:

- gpui: Add linear gradient support to fill background

Run example:

```
cargo run -p gpui --example gradient
cargo run -p gpui --example gradient --features macos-blade
```

## Demo

In GPUI (sRGB):

<img width="761" alt="image"
src="https://github.com/user-attachments/assets/568c02e8-3065-43c2-b5c2-5618d553dd6e">

In GPUI (Oklab):

<img width="761" alt="image"
src="https://github.com/user-attachments/assets/b008b0de-2705-4f99-831d-998ce48eed42">

In CSS (sRGB): 

https://codepen.io/huacnlee/pen/rNXgxBY

<img width="505" alt="image"
src="https://github.com/user-attachments/assets/239f4b65-24b3-4797-9491-a13eea420158">

In CSS (Oklab):

https://codepen.io/huacnlee/pen/wBwBKOp

<img width="658" alt="image"
src="https://github.com/user-attachments/assets/56fdd55f-d219-45de-922f-7227f535b210">


---

Currently only support 2 color stops with linear-gradient. I think this
is we first introduce the gradient feature in GPUI, and the
linear-gradient is most popular for use. So we can just add this first
and then to add more other supports.

crates/gpui/src/scene.rs

@@ -2,8 +2,8 @@
 #![cfg_attr(windows, allow(dead_code))]
 
 use crate::{
-    bounds_tree::BoundsTree, point, AtlasTextureId, AtlasTile, Bounds, ContentMask, Corners, Edges,
-    Hsla, Pixels, Point, Radians, ScaledPixels, Size,
+    bounds_tree::BoundsTree, point, AtlasTextureId, AtlasTile, Background, Bounds, ContentMask,
+    Corners, Edges, Hsla, Pixels, Point, Radians, ScaledPixels, Size,
 };
 use std::{fmt::Debug, iter::Peekable, ops::Range, slice};
 
@@ -458,7 +458,7 @@ pub(crate) struct Quad {
     pub pad: u32, // align to 8 bytes
     pub bounds: Bounds<ScaledPixels>,
     pub content_mask: ContentMask<ScaledPixels>,
-    pub background: Hsla,
+    pub background: Background,
     pub border_color: Hsla,
     pub corner_radii: Corners<ScaledPixels>,
     pub border_widths: Edges<ScaledPixels>,
@@ -671,7 +671,7 @@ pub struct Path<P: Clone + Default + Debug> {
     pub(crate) bounds: Bounds<P>,
     pub(crate) content_mask: ContentMask<P>,
     pub(crate) vertices: Vec<PathVertex<P>>,
-    pub(crate) color: Hsla,
+    pub(crate) color: Background,
     start: Point<P>,
     current: Point<P>,
     contour_count: usize,