Ui::vertical_centered is not centered

[RazrFalcon]

Ui::vertical_centered not always place the widget in the center.

In the demo app, it can be reproduced by resizing the Windows Options dialog.

To Reproduce
Steps to reproduce the behavior:

1. Create a widget using Ui::vertical_centered
2. Resize the parent layout

Expected behavior

The layout content should be centered.

Screenshots

Desktop (please complete the following information):

• OS: Linux

egui version: 07a17dc

[RazrFalcon]

Looks like this is the same as #179

[emilk]

thanks for the report!