Improve editor state initialization

[KoBeWi]

When opening some scene for the first time, the current editor state was leaking into the new scene. This PR ensures that editors call clear() when the scene has no stored state.

It also implements clear() for 2D editor, which resets position and zoom. If fixes an issue where opening some scene could lead to the scene being out of view.

Before

godot.windows.editor.dev.x86_64_aDSn0UJCwk.mp4

After

godot.windows.editor.dev.x86_64_4eurctyyP9.mp4

(other 2D state elements are handled by #74682)

[KoBeWi]

I also changed the default view offset. Previously it was some arbitrary value, now it's centered in the viewport. I pushed it in separate commit for now, because not sure if it's a wanted change.

[akien-mga]

I also changed the default view offset. Previously it was some arbitrary value, now it's centered in the viewport. I pushed it in separate commit for now, because not sure if it's a wanted change.

The underlying idea for the arbitrary offset is that in 2D, your scene "area" will often range from (0, 0) to a point in the (X+, Y+) quadrant.

For example the main scene's root viewport / camera:

On the other hand it's true that for props/characters/etc., you'd typically have them centered on the origin, in which case the default offset isn't the best:

But it's hard to find a balance that fits both types of scenes IMO.

I think it used to be centered in some earlier versions of Godot (2.x maybe?), and I always had to scroll, as the default scroll would basically have the left and top halves of the viewport empty.

[KoBeWi]

I'll revert that change for now and we'll see if it's really a problem.

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.0.3.