LibWeb: Add BorderRadiusCornerClipper

This is a helper class for clipping the corners off a element.

This works in a similar way to how (outline) borders are painted.

The steps are:
  1. A small bitmap that fits only the corners is allocated
  2. The corners are painted into the bitmap
  3. The existing pixels (where the corners will be painted)
     are copied using the (inverse) corner bitmap as a mask
     (done before the element is painted)
  4. The element is painted
  5. The areas outside the corner radii are restored

Like with the borders, this only requires allocation on the first
paint.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -289,6 +289,7 @@ set(SOURCES
     Page/Page.cpp
     Painting/BackgroundPainting.cpp
     Painting/BorderPainting.cpp
+    Painting/BorderRadiusCornerClipper.cpp
     Painting/ButtonPaintable.cpp
     Painting/CanvasPaintable.cpp
     Painting/CheckBoxPaintable.cpp