Re: [fxtf-drafts] [filters] Backdrop filters should not use BackgroundImage

I created a [test case](https://codepen.io/AmeliaBR/pen/deGwNE/). For what it's worth, the Chrome implementation respects isolation.  (On the other hand, the Chrome implementation *doesn't respect border-radius, which is another thing not well defined in the spec.)

Screenshots (the element with the green border has isolation, the circles on the left are using an invert backdrop filter; the circles on the right are using a difference blend mode):

Chrome 65:
![In the unisolated case, both the invert backdrop filter and the difference mix-blend-mode cause the web page background to be inverted from orange circles on light blue to blue circles on brown; for the filter (left) the effect is applied as a square, ignoring the circular border of the test case; in the isolated case, the background is unaffected by either element.](https://user-images.githubusercontent.com/9876129/39157782-aea12be4-4719-11e8-80d7-1d25857017b7.png)

Safari 11:
![The backdrop filter tests invert the background pattern in both cases (and are neatly contained to the border circle in both cases); the blend-mode test only creates an inverting effect on the background when it isn't isolated.](https://s3.amazonaws.com/media.crossbrowsertesting.com/users/97505/snapshots/z54a88c08a0d1108d854.png)


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Received on Monday, 23 April 2018 23:19:56 UTC