[css-anchor-position] Add containing block rules to acceptable elements in top layer

[jamesnw]

Currently, the spec for Finding an anchor only requires that a positioned el is in a higher top layer than possible anchor for an element in a top layer to be an acceptable anchor.

However, there are also containing block rules for the positioned el, even if it is in a higher top layer. For example, a child element of a popover with position: absolute would have the top layer root element (the popover) as a containing block, and would not be able to be positioned. If it is fixed, its containing block would be the viewport, and then can be positioned.

I'd propose changing the point to positioned el is in a higher top layer than possible anchor and positioned el's containing block is the viewport or initial containing block.