Re: [fxtf-drafts] Clip backdrop root (input) image to border box of element (#342)

Here's a testcase which uses `drop-shadow()` in `backdrop-filter` in order to create a glow effect on the background: https://codepen.io/mstange/pen/dBvJRR

<img width="662" alt="Screen Shot 2019-06-21 at 11 20 01 AM" src="https://user-images.githubusercontent.com/961291/59943234-8d295b80-9416-11e9-8bde-54e89666f8ea.png">

It's a bit hard to find appropriate uses for this because `drop-shadow` needs some variation in the input's alpha channel in order to do anything interesting, so you need a Backdrop Image which has some transparent parts, which you can only get if you trigger a non-root Backdrop Root, e.g. using opacity.

In this testcase the glow would look a bit better along the edges if the input was *not* clipped to the element's border box, and if the shadow blur could actually sample from a Backdrop Image that extends beyond the border box. But it's hard to notice the difference.

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