Support custom cursors (images)

[PPakalns]

Support overriding egui cursor default mapping to SystemCursorIcon with custom bevy_winit CursorIcon (e.g. Image)

[OwlyCode]

Best workaround I've found in the meantime is to have my own system run just after process_output_system:

// ...
.add_systems(PostUpdate, custom_cursor.after(process_output_system))
// ...

fn custom_cursor(mut primary_query: Query<&mut CursorIcon, With<PrimaryWindow>>, asset_server: Res<AssetServer>) {
    let Ok(mut cursor_icon) = primary_query.get_single_mut() else {
        return;
    };

    if let CursorIcon::System(_system_icon) = cursor_icon.as_ref() {
        *cursor_icon = CursorIcon::Custom(CustomCursor::Image {
            handle: asset_server.load("ui/arrow.png"),
            hotspot: (0, 0),
        });
    }
}

[vladbat00]

Hi! I'm hesitant to provide a separate API for this, I think custom mapping should be more convenient to implement with user systems, like the one that @OwlyCode suggested. This approach will always be more flexible and expressible imo.
In #388, I also made some changes that make bevy_egui updates of the cursor icon disableable.

Please let me know if this addresses the issue.