Describe the bug

In v0.28.0, the contents of a TopBottomPanel on top are now shifted downwards by 2px, and vertical separators start 2px below where they should, leaving a gap. This can be clearly seen in the egui.rs demo:

For a bottom panel, it's somewhat opposite--items are shifted upwards, and while separators don't leave a gap at the bottom, they do extend too far into the top (note the overdraw between the vertical separator and the horizontal line above:

I believe this may have been caused by #4351, but don't quote me on that.

To Reproduce
Steps to reproduce the behavior:

1. Create an egui app
2. Add a TopBottomPanel
3. Add a button to it
4. Add a vertical separator to it
5. Observe that the elements are off-center

Expected behavior

Vertical separators should span exactly the height of the panel, and elements with a height of interact_size.y should appear centered.