filters/convolve_matrix.rs: use set_attribute()
filters/convolve_matrix.rs: use set_attribute()

I'll deal with the attributes that use NumberOptionalNumber later.

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

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