css: Don't use -gtk-icon-shadow for thumbnails

Gaussian blur has high performance impact. Also, thumbnails with
transparent areas don't get a shadow around their bounding rectangle.

Instead, use gtk_render_background() which is both cheaper and allows
us to use the same CSS from the GTK 4 port. Furthermore, it allows us
to set an  opaque background for thumbnails with transparent parts.

Fixes https://gitlab.gnome.org/GNOME/nautilus/-/issues/2161