feConvolveMatrix: check the kernel_unit_length at render time, not at parse time
feConvolveMatrix: check the kernel_unit_length at render time, not at parse time

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

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