effects_below merged all subpaths into one silhouette and filled it
with the NonZero rule, discarding each subpath's even_odd flag, so the
holes of donut, frame and similar preset shapes filled in under outer
shadows, glows and reflections. Fill each participating subpath under
its own fill rule at the three effect fill sites, mirroring
fill_geometry; the glow stroke keeps the merged silhouette.
Fixes#1596. The picture-content shadow clip path is unchanged
(that is #1597's territory).