sdl2 transparent window setting applies to all elements

gistfile1.txt

fn run(font_path: &Path) -> Result<(), String> {
    let sdl_context = sdl2::init()?;
    let video_subsys = sdl_context.video()?;
    let ttf_context = sdl2::ttf::init().map_err(|e| e.to_string())?;

    let mut window = video_subsys
        .window("SDL2_TTF Example", SCREEN_WIDTH, SCREEN_HEIGHT)
        .position_centered()
        .opengl()
        .build()
        .map_err(|e| e.to_string())?;
    window.set_opacity(0.5)?;

    let mut canvas = window.into_canvas().build().map_err(|e| e.to_string())?;
    let texture_creator = canvas.texture_creator();

    // Load a font
    let mut font = ttf_context.load_font(font_path, 64)?;
    font.set_style(sdl2::ttf::FontStyle::BOLD);

    // render a surface, and convert it to a texture bound to the canvas
    let surface = font
        .render("Hello Rust!")
        .shaded(Color::RGB(255, 0, 0), Color::RGBA(0, 0, 0, 255))
        .map_err(|e| e.to_string())?;

    let texture = texture_creator
        .create_texture_from_surface(&surface)
        .map_err(|e| e.to_string())?;

    canvas.set_draw_color(Color::RGBA(0, 0, 0, 255));
    canvas.clear();

    let TextureQuery { width, height, .. } = texture.query();

    // If the example text is too big for the screen, downscale it (and center irregardless)
    let padding = 64;
    let target = get_centered_rect(
        width,
        height,
        SCREEN_WIDTH - padding,
        SCREEN_HEIGHT - padding,
    );

    canvas.copy(&texture, None, Some(target))?;
    canvas.present();

    'mainloop: loop {
        for event in sdl_context.event_pump()?.poll_iter() {
            match event {
                Event::KeyDown {
                    keycode: Some(Keycode::Escape),
                    ..
                }
                | Event::Quit { .. } => break 'mainloop,
                _ => {}
            }
        }
    }

    Ok(())
}