[whatwg/fullscreen] Clarify backdrop/filter/paint behavior for Fullscreen API (#165)

In separate issue https://github.com/immersive-web/dom-overlays/issues/20, @chrishtr pointed out several issues related to the interaction of the fullscreen API and WebXR DOM Overlays, and suggested several changes to the Fullscreen API to better define the expected behavior:

@chrishtr wrote:
> I researched the fullscreen and DOM overlay specs in some detail, together with @progers and @mfreed7. @cdata thanks also for the links and comments, they were very helpful.
> 
> I think there is a simple way forward, via a few small spec tweaks to the fullscreen and WebXR DOM overlay specs.
> 
> 1. [Fullscreen spec] Specify that each element in a [top layer](https://fullscreen.spec.whatwg.org/#top-layer):
> 
> * is a [backdrop root](https://drafts.fxtf.org/filter-effects-2/#backdrop-root)
> * has [`contain:paint`](https://www.w3.org/TR/css-contain-1/#valdef-contain-paint)
> 
> 1. [Fullscreen spec] Clarify that there is a "root stacking context" above the "root element stacking context", whose children are the root element stacking context and the top layer elements' stacking contexts, in that order. The root stacking context has simplified behavior: it paints the root element stacking context, then the stacking contexts for the elements in the top layer.
> 2. [WebXR spec] Specify that in WebXR DOM overlay mode, the root element stacking context does not paint.
> 3. [Fullscreen spec] Specify that the viewport does not scroll overflow when elements are present in the top layer.
> 
> Item 1 solves the problem of backdrop-filter being ill-defined and the top layer element failing to be a containing block for `position: fixed` descendants otherwise, which is bad because it makes the position of those elements ill-defined.
> 
> Item 2 results in a spec that makes clear that no transforms, filters, etc. on any DOM elements not in the overlay subtree apply to it, _including_ the HTML element.
> 
> Items 3 (pus item 2) solves the problem of ensuring that, in WebXR DOM overlay mode, content outside of the overlay does not paint anything. This is necessary because the backdrop of the overlay needs to be an XR scene, not the other DOM content. In fullscreen mode, OTOH, there is by default an opaque backdrop to the fulllscreen element, but if you manually override its transparency, you can see the rendering output of other elements.
> 
> Item 4 avoids scrollbars occurring on the page due to content behind the overlay having scrolling overflow.



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