Bug 317059

The `backdrop-filter` CSS property accepts the same `<filter-value-list>` grammar as `filter`, including `<url>` references to SVG `<filter>` elements (Filter Effects 2 for The 'backdrop-filter' property). WebKit currently silently drops the filter when the chain contains a `url(...)` reference (bug 245510); the existing WPT coverage in `css/filter-effects/` is thin and several spec-prescribed cases have no test at all. This bug tracks adding new WPT tests that cover the reference-filter gaps. It is filed separately from bug 245510 so the test patch can land independently of the implementation work. Specs: * https://drafts.csswg.org/filter-effects-1/ * https://drafts.csswg.org/filter-effects-2/ MISSING TESTS 1. Chained `url()` references aka `backdrop-filter: url(#a) url(#b)` 2. Mixing `<filter-function>` with `<url>` aka `backdrop-filter: invert(1) url(#identity)` 3. `url()` to a non-`<filter>` element. 4. `feDisplacementMap`. 5. `filterUnits="userSpaceOnUse"` with an explicit primitive region.