Twenty Fourteen: change diagonal lines placeholder background-attachment

[buffler]

Something that can be a bit jarring about small diagonal line patterns (like those used as a background for featured images in Twenty Fourteen) is the way they'll appear to flash as you scroll down the page.

On any site that makes good use of featured images, ​which the diagonal lines encourage, this won't be noticed often. But on any site that fails to make good use of featured images, the flashing-while-scrolling thing may prove distracting to its visitors.

Maybe this is really trivial? - but the fix is easy. Just adding background-attachment: fixed; to the elements that use these backgrounds will achieve the same effect but with improved results: you still have featured images (or lack thereof) with the diagonal background appearing to scroll up the page, just no flashing.

I've attached a patch (hope I do this right, it's my first).

[sabreuse]

related: #25300

[celloexpressions]

This makes a big difference, should definitely go in regardless of the decision in #25300, I think (although the patches will conflict so should maybe be merged).

[lancewillett]

In 25610:

Twenty Fourteen: redo the featured image background pattern with CSS plus minor style adjustments. Props iamtakashi, fixes #25300. This also fixes #25363, props buffler.