LibWeb: An initial implementation of `backdrop-filter`

[MacDue]

This is an initial implementation of the backdrop-filter CSS property, there's still a few missing bits and a bunch of new FIXMEs, but it seems to work fairly well. See commits or the pretty pictures below:

LibWeb:

Firefox:

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Performance with a backdrop-filter also seems okay for the most part. In the clip below I'm randomly scrolling up/down on a page with a grayscale() invert() blur(2px) filter on the header, and it does not feel that slow. In profiling it seems that about 10% of the time is spend doing the filtering (which I don't think is that bad).

screen-capture - 2022-09-03T155155.178.webm

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Apologies for the girth of this PR