css: Avoid excessive shadow rendering

css: Avoid excessive shadow rendering

The shadow rendering code had code to exit early if we determine
that the shadow is entirely clipped away. Unfortunately, the check
based on cairo clip extents fails for any clip regions that are
more complicated than axis-aligned rectangles, and we are using
a hollow rounded rectangle here. So, instead, do the check manually,
using the just-introduced API in GtkRoundedBox.