LibWeb: Paint backdrop-filter effects!

This implements all the filters other than `saturate()`,
`hue-rotate()`, and `drop-shadow()`.

There are still a lot of FIXMEs to handle in the actual implementation
though, particularly around supporting transforms, but this handles
the most common use cases :^)

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -335,6 +335,7 @@ set(SOURCES
     Painting/CanvasPaintable.cpp
     Painting/CheckBoxPaintable.cpp
     Painting/GradientPainting.cpp
+    Painting/FilterPainting.cpp
     Painting/ImagePaintable.cpp
     Painting/InlinePaintable.cpp
     Painting/LabelablePaintable.cpp