Add macOS-specific has_shadow and with_has_shadow to ViewportBuilder (#6850)

* [X] I have followed the instructions in the PR template

This PR fixes a ghosting issue I encountered while making a native macOS
transparent overlay app using egui and eframe by exposing the [existing
macOS window attribute
`has_shadow`](https://docs.rs/winit/latest/winit/platform/macos/trait.WindowExtMacOS.html#tymethod.has_shadow)
to the `ViewportBuilder` via a new `with_has_shadow` option.

## Example of Ghosting Issue

### Before `ViewportBuilder::with_has_shadow`
By default, the underlying `winit` window's `.has_shadow()` defaults to
`true`.


https://github.com/user-attachments/assets/c3dcc2bd-535a-4960-918e-3ae5df503b12

### After `ViewportBuilder::with_has_shadow`


https://github.com/user-attachments/assets/484462a1-ea88-43e6-85b4-0bb9724e5f14

Source code for the above example can be found here:

https://github.com/gaelanmcmillan/egui-overlay-app-with-shadow-artifacts-example/blob/main/src/main.rs


### Further background
By default on macOS, `winit` windows have a drop-shadow effect. When
creating a fully transparent overlay GUI, this drop-shadow can create a
ghosting effect, as the window content has a drop shadow which is not
cleared by the app itself. This issue has been experienced by users of
`bevy`, another Rust project that has an upstream dependency on `winit`:
https://github.com/bevyengine/bevy/issues/18673

crates/egui-winit/src/lib.rs

@@ -1620,6 +1620,7 @@ pub fn create_winit_window_attributes(
         title_shown: _title_shown,
         titlebar_buttons_shown: _titlebar_buttons_shown,
         titlebar_shown: _titlebar_shown,
+        has_shadow: _has_shadow,
 
         // Windows:
         drag_and_drop: _drag_and_drop,
@@ -1764,7 +1765,8 @@ pub fn create_winit_window_attributes(
             .with_titlebar_buttons_hidden(!_titlebar_buttons_shown.unwrap_or(true))
             .with_titlebar_transparent(!_titlebar_shown.unwrap_or(true))
             .with_fullsize_content_view(_fullsize_content_view.unwrap_or(false))
-            .with_movable_by_window_background(_movable_by_window_background.unwrap_or(false));
+            .with_movable_by_window_background(_movable_by_window_background.unwrap_or(false))
+            .with_has_shadow(_has_shadow.unwrap_or(true));
     }
 
     window_attributes