LibWeb: Plumb style/computed values for backdrop-filter

Author: https://github.com/MacDue
Commit: ec4de1e07d
Pull-request: https://github.com/SerenityOS/serenity/pull/15123
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/gmta
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibWeb/CSS/StyleProperties.h

@@ -70,6 +70,7 @@ public:
     Optional<CSS::AlignItems> align_items() const;
     Optional<CSS::AlignSelf> align_self() const;
     Optional<CSS::Appearance> appearance() const;
+    CSS::BackdropFilter backdrop_filter() const;
     float opacity() const;
     Optional<CSS::Visibility> visibility() const;
     Optional<CSS::ImageRendering> image_rendering() const;