[css-view-transitions-1] Consider a wider margin from viewport than 0 for offscreen elements

[noamr]

As per this resolution, we display the live snapshot instead of the old one if the element in the old state was captured when out of the viewport.

Concerns were raised that this would make some legitimate use cases, such as crossfading in things that are close to the viewport, is going to be broken by this.

The proposal is to have a standard margin for this that is >0 (perhaps 50vmax?).