shadow-box: Bail out blur early if radius is 1px

shadow-box: Bail out blur early if radius is 1px

For radius 1px the current implementation rounds down to a 1 px box
filter which is a no-op. Rather than creating useless shadow masks
in this case we bail out blurring early.

Another alternative would be to make radius 1px round up to a 2 px box
filter, but that would change the rendering of Adwaita which is probably
not a great idea this late in the cycle.

https://bugzilla.gnome.org/show_bug.cgi?id=746468