Screenshots use previous camera position

## Bevy version

0.15.0

## What you did

In `PostUpdate`, when user triggers a screenshot:

```rust
let (mut proj, mut transform) = icon_camera.single_mut(); // icon_camera is Query<(&mut OrthographicProjection, &mut Transform), (With, Without, Without)>

let (center, size) = get_camera_parameters(aabb);

transform.translation = Vec3::new(center.x, center.y, 0.);
let size_max = size.x.max(size.y);

proj.scaling_mode = ScalingMode::Fixed{width: size_max, height: size_max};
//proj.scale = size_max / 512.0;

proj.area = Rect::from_center_size(bevy::math::Vec2::new(center.x, center.y), bevy::math::Vec2::new(size_max, size_max));
println!("pos {}, size {}", center, size_max);

commands
.spawn(Screenshot::image(image.clone()))
.observe(bevy::render::view::screenshot::save_to_disk(path));
```

I'm setting a camera position and taking a screenshot when the user saves an object in my game

## What went wrong

When I trigger the screenshot once and look at the saved image, it isn't at the right position, but instead the initial position of the icon camera (from when I created it in `PreStartup`.

Then, when I trigger it a second time, it's now at the right position, as long as the object didn't move. Each screenshot uses the previous camera position, instead of what I just changed it to

I think this is a bug, but it could also be intended behaviour? Does Bevy want me to wait a frame for the image to update?