Scour tried to handle "order" attribute as a SVGLength. However, the
"order" attribute *can* consist of two integers according to the
[SVG 1.1 Specification] and SVGLength is not designed to handle that.
With this change, we now pretend that "order" is a string, which side
steps this issue.
[SVG 1.1 Specification]: https://www.w3.org/TR/SVG11/single-page.html#filters-feConvolveMatrixElementOrderAttributeCloses: #189
Signed-off-by: Niels Thykier <niels@thykier.net>