Immediate viewport would crash with ViewportBuilder settings fullscreen + transparent + always_on_top

[wangxiaochuTHU]

Describe the bug
The whole app would crash when showing a Immediate viewport with fullscreen + transparent + always_on_top set true simultaneously.

To Reproduce

Just set the ViewportBuilder to enable fullscreen + transparent + always_on_top all together, and the whole app would crash.

PS: The crash would happen ONLY WHEN these THREE attributes are enabled AT THE SAME TIME.

impl eframe::App for MyApp {
    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
            ctx.show_viewport_immediate(
                egui::ViewportId::from_hash_of("immediate_viewport"),
                egui::ViewportBuilder::default()
                    .with_fullscreen(self.fullscreen)
                    .with_transparent(true)
                    .with_always_on_top(),
                |ctx, class| {
                    assert!(
                        class == egui::ViewportClass::Immediate,
                        "This egui backend doesn't support multiple viewports"
                    );
                },
            );
   }
}

Expected behavior
No crash.

Desktop (please complete the following information):

• OS: Windows
• Version 0.26.2