Fix issue where nested side panels can't be resized larger than their contents

[teddemunnik]

This fixes an issue where a panel could not be resized to a size larger than it's content layout rect when using nested panels. The issue shows both when nesting panels in panels, and when nesting panels in a window. I have created a github project as a repro here:
https://github.com/teddemunnik/egui_inner_panel_repro/blob/master/src/main.rs

egui_bug_before.mp4

egui_bug_after.mp4

• I have followed the instructions in the PR template

[emilk]

When testing, let's make sure this doesn't break the auto-sizing of panels

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5186-pr/fix_nested_side_panel
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[teddemunnik]

When testing, let's make sure this doesn't break the auto-sizing of panels

It probably does! My bad, also found the docs regarding this behavior on panels when looking for the auto sizing behavior as you mentioned it