Split out the blurred shadow rendering in three steps:
- creation of a surface of the appropriate size - we use the clip
  rectangle as a good measurement for the size, since we won't render
  out of it anyway
- painting the unblurred shape on the surface - this is responsibility
  of the single shadow implementations
- blur the surface and compose the result back on the original cairo_t

This means we can share code between the implementations for the first
and third steps; it also makes the code independent of the rendered
size, so we can avoid passing down a cairo_rectangle_t with e.g. the
icon coordinates.