gpui: Add opacity to support transparency of the entire element (#17132)

Release Notes:

- N/A

---

Add this for let GPUI element to support fade in-out animation.

## Platform test

- [x] macOS
- [x] blade `cargo run -p gpui --example opacity --features macos-blade`

## Usage

```rs
div()
    .opacity(0.5)
    .bg(gpui::black())
    .text_color(gpui::black())
    .child("Hello world")
```

This will apply the `opacity` it self and all children to use `opacity`
value to render colors.

## Example

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

<img width="612" alt="image"
src="https://github.com/user-attachments/assets/f1da87ed-31f5-4b55-a023-39e8ee1ba349">

crates/gpui/src/scene.rs

@@ -640,16 +640,19 @@ impl From<MonochromeSprite> for Primitive {
     }
 }
 
-#[derive(Clone, Debug, Eq, PartialEq)]
+#[derive(Clone, Debug, PartialEq)]
 #[repr(C)]
 pub(crate) struct PolychromeSprite {
     pub order: DrawOrder,
+    pub pad: u32, // align to 8 bytes
     pub grayscale: bool,
+    pub opacity: f32,
     pub bounds: Bounds<ScaledPixels>,
     pub content_mask: ContentMask<ScaledPixels>,
     pub corner_radii: Corners<ScaledPixels>,
     pub tile: AtlasTile,
 }
+impl Eq for PolychromeSprite {}
 
 impl Ord for PolychromeSprite {
     fn cmp(&self, other: &Self) -> std::cmp::Ordering {