The main window is not centered correctly when the scale factor is not 1.

[zhatuokun]

Describe the bug The main window is not centered correctly when the scale factor is not 1(my default 1.25).

To Reproduce Steps to reproduce the behavior:

1. cargo new example
2. add eframe = "0.20.0"
3. run the following code

fn main() {
    eframe::run_native(
        "app_name",
        eframe::NativeOptions {
            centered: true,
            initial_window_size: Some(eframe::egui::vec2(960.0, 670.0)),
            ..Default::default()
        },
        Box::new(|_| Box::new(Application::default())),
    );
}

#[derive(Default)]
struct Application;

impl eframe::App for Application {
    fn update(&mut self, ctx: &eframe::egui::Context, _frame: &mut eframe::Frame) {
        eframe::egui::CentralPanel::default().show(ctx, |ui| {
            ui.heading("Hello World");
        });
    }
}

Expected behavior The window is centered correctly.

Desktop (please complete the following information):

• OS: Windows10
• Version: 21H2