From [the spec](https://www.w3.org/TR/css-backgrounds-3/#shadow-blur):
> the resulting shadow must approximate (with each pixel being within 5%
> of its expected value) the image that would be generated by applying to
> the shadow a Gaussian blur with a standard deviation [sigma] equal to
> half the blur radius
This does not change the value of `sigma`, it only corrects the value of
`sampling_radius` used to size the shadow texture. Since the texture is
no longer being slightly oversized it won't get scaled down when rendered
according to the dimensions provided by `st_shadow_get_box` in
`_st_paint_shadow_with_opacity`.
Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4409
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1897>
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