shadow-factory: Make sure to never use a negative spread

The smallest possible spread corresponds to an unblurred shadow, which
neither grows nor shrinks - thus the spread should be zero not negative
as returned by our current calculation.

https://bugzilla.gnome.org/show_bug.cgi?id=731353
meta-window-actor: Don't add shadows to popups if the toolkit already does

GTK+ will add its own shadows client-side, so we don't need to anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=731353