Fix the initial value of feSpotSlight's specularExponent
Fix the initial value of feSpotSlight's specularExponent

https://drafts.fxtf.org/filter-effects/#element-attrdef-fespotlight-specularexponent

It is supposed to be 1, but due to #[derive(Default)] it was being set
to 0.

Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/763>

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg