Since these part really are the same in all of the x or y direction
and we don't blur in that direction we can just blur one line and
repeat it during drawing.
https://bugzilla.gnome.org/show_bug.cgi?id=746468
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