Window centering does not respect scale factor override

## Bevy version

0.11.2

## \[Optional\] Relevant system information

- macOS 13.5
- Apple M1 Max (Metal)

```ignore
`AdapterInfo { name: "Apple M1 Max", vendor: 0, device: 0, device_type: IntegratedGpu, driver: "", driver_info: "", backend: Metal }`
```

## What you did

1. Create a basic Bevy application with `DefaultPlugins`
2. Override the `primary_window` to 640x360 in the `WindowPlugin` initialization
3. Add `Startup` system for `setup_resolution` that adjusts `window.resolution.set_scale_factor_override`
4. Call `window.position.center(MonitorSelection::Current)` to center the window
5. Observe incorrect centering using `scale_factor` instead of override

## What went wrong

The window should be centered using the new `scale_factor_override` instead of the current `scale_factor` from the OS.

Ex:

```rust
pub fn setup_resolution(mut windows: Query<&mut Window>) {
let mut window = windows.single_mut();

window.resolution.set_scale_factor_override(Some(3.0));
window.position.center(MonitorSelection::Current);

info!("Window created: {:?}", window.resolution);
}
```

```ignore
`Window created: WindowResolution { physical_width: 1920, physical_height: 1080, scale_factor_override: Some(3.0), scale_factor: 2.0 }`
```

## Additional information

Only tested on macOS currently, not sure if works properly on Windows due to how scale factor works.

I encountered this by working on a pixel-art game with a reference resolution of 640x360 and want to apply a scale factor to retain the sharpness via integer scaling (1x, 2x, 3x, 4x, etc). I don't know if this the correct way, but it did expose this bug.

Start by tracing the Window position.center(MonitorSelection::Current) path and how it reads scale_factor versus scale_factor_override. Reproduce the macOS example with a 640x360 window and an override of 3.0; done means centering uses the override while preserving correct behavior for platforms and windows without one.

Written by the indexing model from the issue text.