This PR fixes an issue where a bad pivot stored in app state would
override the pivot actually set from code. The root code of this issue
if the `Window`'s `State` containing `pivot` in the first place, as
`pivot` is not part of a state to be tracked. It makes the `State`
structure more ergonomic though, so this PR leaves the `pivot` field but
always overrides it with the value provided by user code.
#### Repro of the original issue
This issue can be reproduced using the `re_ui_example` app from the
Rerun repo, at this commit:
fb5add0047.
By using this `app.ron` file, the bug will appear:
```
{
"egui": "(
areas:{
((0)):(
areas:{
(13430889033718688666):(pivot_pos:(x:565.5,y:328.0),pivot:((Min,Min)),size:(x:364.0,y:75.5),interactable:true),
},
)
}
)",
}
```
The modal is entered based on it's top-left corner even though the code
actually specifies a center-center pivot:
<img width="1312" alt="image"
src="https://github.com/emilk/egui/assets/49431240/6df1a8bb-d1ea-4e15-866c-5f14138dda0e">
With this PR, the centring is correct even with the "poisoned"
`app.ron`:
<img width="1312" alt="image"
src="https://github.com/emilk/egui/assets/49431240/7f50b2d4-9245-48f3-91de-c747bbc8c40e">
365a8d2240