Hidden (yet non-occluded) images should get fetchpriority=low

[westonruter]

This is split off from #1309. See specifically #1309 (comment).

After #1482 there are the following scenario exists where fetchpriority=low can still be added:

• If an element has a non-zero intersectionRatio and the element has a computed style of visibility:hidden or opacity:0, then it should get fetchpriority=low.

See the comment for discussion about what to do about images with a computed style of display:none.