Fix Window positioning bug when bad pivot is stored in app data

[abey79]

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: rerun-io/rerun@fb5add0. 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:

With this PR, the centring is correct even with the "poisoned" app.ron:

[abey79]

The fact that persisted app state can corrupt State makes me think that this "TODO" comment might be incorrect:

egui/crates/egui/src/containers/area.rs

Lines 7 to 23 in 83f5d5c

	/// State that is persisted between frames.
	// TODO(emilk): this is not currently stored in `Memory::data`, but maybe it should be?
	#[derive(Clone, Copy, Debug)]
	#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
	pub(crate) struct State {
	/// Last known pos of the pivot
	pub pivot_pos: Pos2,
	pub pivot: Align2,
	/// Last know size. Used for catching clicks.
	pub size: Vec2,
	/// If false, clicks goes straight through to what is behind us.
	/// Good for tooltips etc.
	pub interactable: bool,
	}

[emilk]

The fact that persisted app state can corrupt State makes me think that this "TODO" comment might be incorrect:

The comment is correct. The area state is not stored in Memory::data, but in another part of Memory (so it is still persisted). Functionally this matters little, it is just inconsistent with other widget state.