Add Context::set_cursor_image for OS-level custom cursors (#8155)

## What

Adds a way for apps to push an RGBA bitmap as the OS cursor — the
missing companion to `Context::set_cursor_icon`. The integration
translates it into a real `winit::CustomCursor`, so the cursor is drawn
by the compositor and can extend past the egui window edge like any
native cursor.

## Why

Apps with custom-shaped windows (Winamp-style skins, themed launchers,
kiosk apps) currently have no clean way to display a custom cursor:
- `CursorIcon` is limited to the standard system enum.
- Painting the cursor sprite via `egui::Painter` works inside the canvas
but gets clipped at the window edge — the bottom/right of the cursor
disappears the moment the pointer is near the boundary, and there's no
way to render onto the desktop area exposed by a
transparent/region-shaped window.

`winit` 0.30+ already supports `CustomCursor::from_rgba` +
`ActiveEventLoop::create_custom_cursor`, but `egui-winit` doesn't
surface it. This PR exposes it through egui.

### Visual demonstration

Driving use case: a Winamp WSZ skin player
([all3f0r1/oneamp](https://github.com/all3f0r1/oneamp)) with a
transparent + region-shaped window where the skin ships its own `.cur`
files. The bottom-right corner of the playlist exposes the resize cursor
— notice how it gets clipped at the window edge in the painter-based
approach.

| Before (cursor painted via `egui::Painter`) | After (cursor pushed via
`set_cursor_image`) |
| --- | --- |
| ![cursor clipped at the bottom-right of the playlist
window](https://raw.githubusercontent.com/all3f0r1/egui/pr-assets/cursor-clipping-before.png)
| ![cursor extends cleanly past the window edge onto the
desktop](https://raw.githubusercontent.com/all3f0r1/egui/pr-assets/cursor-clipping-after.png)
|

## API

```rust
// new in egui::data::output
pub struct CustomCursorImage {
    pub rgba: std::sync::Arc<[u8]>,
    pub size: [u16; 2],     // matches winit's u16 to avoid lossy casts
    pub hotspot: [u16; 2],
}

// new field on PlatformOutput (skipped from serde — ephemeral)
pub cursor_image: Option<CustomCursorImage>,

// new method on Context
ctx.set_cursor_image(Some(image)); // overrides cursor_icon for this frame
ctx.set_cursor_image(None);        // revert to cursor_icon
```

`Arc<[u8]>` is intentional: the integration dedupes by `Arc::as_ptr`, so
reusing the same Arc across frames means the bitmap is only uploaded to
the OS once per skin, not once per frame.

## Integration changes

- `egui_winit::State::handle_platform_output_with_event_loop(window,
Option<&ActiveEventLoop>, ...)` is a new method that threads the active
event loop so it can call `event_loop.create_custom_cursor(...)`.
- The legacy `handle_platform_output(window, ...)` delegates with `None`
and silently drops `cursor_image`. **No existing callers break.**
- The icon and bitmap paths are unified in a private `apply_cursor`. The
no-flicker dedupe of the old `set_cursor_icon` is preserved on both
paths.
- If `CustomCursor::from_rgba` rejects the bitmap (bad dimensions,
hotspot OOB, etc.), we log a warning and fall back to the icon path.
- eframe's wgpu + glow integrations thread `&ActiveEventLoop` through
`run_ui_and_paint` (glow already had it; wgpu needed one extra
parameter) and call the new method.
- Immediate viewports keep the old path because they're invoked from a
`Context` callback that doesn't have an event loop reference. Custom
cursors are a no-op in immediate viewports — acceptable since they're a
niche path.

## Fallback semantics

| backend / context              | what happens                  |
|--------------------------------|-------------------------------|
| eframe wgpu/glow main viewport | bitmap displayed via OS       |
| eframe immediate viewport      | falls back to `cursor_icon`   |
| eframe web                     | falls back to `cursor_icon`   |
| custom integrations not opted in | falls back to `cursor_icon` |
| `from_rgba` returns `BadImage` | warning + falls back to icon  |

## Verification

- `cargo fmt --all -- --check` ✅
- `cargo clippy -p egui -p egui-winit -p eframe --all-targets
--all-features -- -D warnings` ✅
- `cargo doc --lib --no-deps -p egui -p egui-winit -p eframe
--all-features` ✅
- `cargo check -p egui --no-default-features --features serde` ✅
(validates the `serde(skip)` on `cursor_image`)
- Interactive validation on Linux/Wayland with the OneAmp WSZ skin
player — see screenshots above.

I haven't run the full snapshot test suite (`scripts/check.sh`) because
we're on Linux and the snapshots are macOS-rendered — happy to run it if
you'd like.

## Notes

Drafted per the contributing guide ("open a draft PR, you may get
helpful feedback early"). Open to design feedback on:
1. Whether `CustomCursorImage` should live in `egui::viewport` rather
than `egui::data::output`.
2. Whether the legacy `handle_platform_output` should grow `event_loop`
directly (breaking) instead of getting a sibling method (non-breaking,
what I did).
3. Whether to also wire it through eframe-web (probably not —
`wasm-bindgen-cursor` would need its own path).

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/eframe/src/native/glow_integration.rs

@@ -679,7 +679,7 @@ impl GlowWinitRunning<'_> {
         let gl_surface = viewport.gl_surface.as_ref().unwrap();
         let egui_winit = viewport.egui_winit.as_mut().unwrap();
 
-        egui_winit.handle_platform_output(&window, platform_output);
+        egui_winit.handle_platform_output_with_event_loop(&window, event_loop, platform_output);
 
         if is_visible {
             let clipped_primitives = integration.egui_ctx.tessellate(shapes, pixels_per_point);

crates/eframe/src/native/wgpu_integration.rs

@@ -409,7 +409,7 @@ impl WinitApp for WgpuWinitApp<'_> {
         self.initialized_all_windows(event_loop);
 
         if let Some(running) = &mut self.running {
-            running.run_ui_and_paint(window_id)
+            running.run_ui_and_paint(window_id, event_loop)
         } else {
             Ok(EventResult::Wait)
         }
@@ -569,7 +569,11 @@ impl WgpuWinitRunning<'_> {
     }
 
     /// This is called both for the root viewport, and all deferred viewports
-    fn run_ui_and_paint(&mut self, window_id: WindowId) -> Result<EventResult> {
+    fn run_ui_and_paint(
+        &mut self,
+        window_id: WindowId,
+        event_loop: &ActiveEventLoop,
+    ) -> Result<EventResult> {
         profiling::function_scope!();
 
         let Some(viewport_id) = self
@@ -710,7 +714,7 @@ impl WgpuWinitRunning<'_> {
             return Ok(EventResult::Wait);
         };
 
-        egui_winit.handle_platform_output(window, platform_output);
+        egui_winit.handle_platform_output_with_event_loop(window, event_loop, platform_output);
 
         let vsync_secs = if is_visible {
             let clipped_primitives = egui_ctx.tessellate(shapes, pixels_per_point);

crates/eframe/src/web/app_runner.rs

@@ -368,7 +368,8 @@ impl AppRunner {
         let egui::PlatformOutput {
             commands,
             cursor_icon,
-            events: _,                    // already handled
+            cursor_image: _, // TODO(alextournai): support custom bitmap cursors on the web (via CSS `url(...)`)
+            events: _,       // already handled
             mutable_text_under_cursor: _, // TODO(#4569): https://github.com/emilk/egui/issues/4569
             ime,
             accesskit_update: _,        // not currently implemented