Appends an outset shadow node around the box given by outline.
| this |
a `GtkSnapshot` |
| outline |
outline of the region surrounded by shadow |
| color |
color of the shadow |
| dx |
horizontal offset of shadow |
| dy |
vertical offset of shadow |
| spread |
how far the shadow spreads towards the outside |
| blur_radius |
how much blur to apply to the shadow |