blur-effect: Use sigma value instead of blur radius

blur-effect: Use sigma value instead of blur radius

Almost all implementations, including CSS [1] of gaussian blurs use the
sigma/standard deviation value as the input parameter, even if they call
that value "radius". Since using sigma is more correct mathematically
and avoids confusion for people used to other blur implementations, use
that parameter here, too.

[1] https://www.w3.org/TR/filter-effects-1/#funcdef-filter-blur

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/991