css shadow: Add extra slop to the blur clipping
css shadow: Add extra slop to the blur clipping

Turns out our blurring function isn't very nice, it has a lot
of energy past the blur radius, so clipping at exactly the
blur radius causes ugly gradient stops. This just adds 4
extra pixels of slop, which makes this better in most cases.

Sidnioulz/SandboxGtk,davidgumberg/gtk,jessevdk/gtk,jigpu/gtk,chergert/gtk,Sidnioulz/SandboxGtk,chergert/gtk,chergert/gtk,jigpu/gtk,davidgumberg/gtk,Lyude/gtk-,jessevdk/gtk,chergert/gtk,alexlarsson/gtk,Sidnioulz/SandboxGtk,Distrotech/gtk2,Sidnioulz/SandboxGtk,alexlarsson/gtk,alexlarsson/gtk,Adamovskiy/gtk,msteinert/gtk,Lyude/gtk-,jessevdk/gtk,chergert/gtk,Distrotech/gtk2,ahodesuka/gtk,Lyude/gtk-,bratsche/gtk-,alexlarsson/gtk,grubersjoe/adwaita,Lyude/gtk-,davidgumberg/gtk,jigpu/gtk,chergert/gtk,grubersjoe/adwaita,jigpu/gtk,alexlarsson/gtk,ahodesuka/gtk,Lyude/gtk-,Distrotech/gtk2,bratsche/gtk-,ebassi/gtk,Adamovskiy/gtk,bratsche/gtk-,davidgumberg/gtk,jadahl/gtk,davidgumberg/gtk,ebassi/gtk,jessevdk/gtk,grubersjoe/adwaita,jigpu/gtk,alexlarsson/gtk,ebassi/gtk,jigpu/gtk,alexlarsson/gtk,grubersjoe/adwaita,ahodesuka/gtk,grubersjoe/adwaita,ahodesuka/gtk,jigpu/gtk,bratsche/gtk-,Adamovskiy/gtk,grubersjoe/adwaita,grubersjoe/adwaita,msteinert/gtk,jadahl/gtk,jessevdk/gtk,davidgumberg/gtk,jadahl/gtk,ebassi/gtk,ebassi/gtk,jadahl/gtk,Sidnioulz/SandboxGtk,jessevdk/gtk,ahodesuka/gtk,ahodesuka/gtk,jadahl/gtk,msteinert/gtk,jadahl/gtk,Adamovskiy/gtk,ahodesuka/gtk,davidgumberg/gtk,Lyude/gtk-,Adamovskiy/gtk,Adamovskiy/gtk,chergert/gtk,chergert/gtk,alexlarsson/gtk,Lyude/gtk-,davidgumberg/gtk,msteinert/gtk,Sidnioulz/SandboxGtk,jessevdk/gtk,jadahl/gtk,bratsche/gtk-,ebassi/gtk,Distrotech/gtk2,jadahl/gtk,Adamovskiy/gtk,Distrotech/gtk2,grubersjoe/adwaita,msteinert/gtk,Adamovskiy/gtk,jigpu/gtk,ahodesuka/gtk,Lyude/gtk-,bratsche/gtk-,msteinert/gtk,Distrotech/gtk2