cssshadowvalue: Apply device scale to the offset when blurring text
Although the surface already has the device scale set, that doesn't apply to the offset which is in raw device units. So we need to scale the device offset explicitly.
Fixes: https://gitlab.com/rastersoft/desktop-icons-ng/-/issues/119
I'm proposing this to GTK3 only because the affected code doesn't exist in GTK4.