There is no need to e.g. blur in the x-direction for the top part of a box shadow. Also, there is no need to extend the mask in the non-blurred direction. https://bugzilla.gnome.org/show_bug.cgi?id=746468
This just creates a large cairo surface and times bluring it at various values of radius. https://bugzilla.gnome.org/show_bug.cgi?id=746468