Add Loading and Fallback States to Image Elements (via StyledImage) (#20371)

@iamnbutler edit:

This pull request enhances the image element by introducing the ability
to display loading and fallback states.

Changes:

- Implemented the loading and fallback states for image elements using
`.with_loading` and `.with_fallback` respectively.
- Introduced the `StyledImage` trait and `ImageStyle` to enable a fluent
API for changing image styles across image types (`Img`,
`Stateful<Img>`, etc).

Example Usage:

```rust
fn loading_element() -> impl IntoElement {
    div().size_full().flex_none().p_0p5().rounded_sm().child(
        div().size_full().with_animation(
            "loading-bg",
            Animation::new(Duration::from_secs(3))
                .repeat()
                .with_easing(pulsating_between(0.04, 0.24)),
            move |this, delta| this.bg(black().opacity(delta)),
        ),
    )
}

fn fallback_element() -> impl IntoElement {
    let fallback_color: Hsla = black().opacity(0.5);

    div().size_full().flex_none().p_0p5().child(
        div()
            .size_full()
            .flex()
            .items_center()
            .justify_center()
            .rounded_sm()
            .text_sm()
            .text_color(fallback_color)
            .border_1()
            .border_color(fallback_color)
            .child("?"),
    )
}

impl Render for ImageLoadingExample {
    fn render(&mut self, _cx: &mut ViewContext<Self>) -> impl IntoElement {
        img("some/image/path")
            .id("image-1")
            .with_fallback(|| Self::fallback_element().into_any_element())
            .with_loading(|| Self::loading_element().into_any_element())
    }
}
```

Note:

An `Img` must have an `id` to be able to add a loading state.

Release Notes:

- N/A

---------

Co-authored-by: nate <nate@zed.dev>
Co-authored-by: michael <michael@zed.dev>
Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/gpui/src/svg_renderer.rs

@@ -10,7 +10,7 @@ pub(crate) struct RenderSvgParams {
 }
 
 #[derive(Clone)]
-pub(crate) struct SvgRenderer {
+pub struct SvgRenderer {
     asset_source: Arc<dyn AssetSource>,
 }
 
@@ -24,7 +24,7 @@ impl SvgRenderer {
         Self { asset_source }
     }
 
-    pub fn render(&self, params: &RenderSvgParams) -> Result<Option<Vec<u8>>> {
+    pub(crate) fn render(&self, params: &RenderSvgParams) -> Result<Option<Vec<u8>>> {
         if params.size.is_zero() {
             return Err(anyhow!("can't render at a zero size"));
         }