clutter: Pass 'radius' to public blur APIs instead of 'sigma'

Although they're in the same units, radius is easier to understand than sigma and makes the public API independent of the blur algorithm used behind the scenes. We now only keep the sigma terminology where the implementation is Gaussian-specific.

The assumption that sigma = radius / 2.0 is actually not new here. We just move it from _st_create_shadow_pipeline (gnome-shell!1905 (merged)) into clutter_blur_new.

Requires gnome-shell!1905 (merged).