This commit adds support for using all your favorite border radii with
box-shadow, that is elliptical, circular, rounded rectangle etc. :^)
There is some work needed to make this more performant. The larger
your border radius is the larger the corner bitmap needs to be,
which means more time spent in FastBoxBlurFilter. There are probably
some tricks to bring this down.
Fixes#14325
ff1e61bd11