Support <filter-value-list> from SVG2
SVG 2 has completely deferred filter effects to the Filter Effects Module Level 1 specification. In addition to being a property (see #614 (closed)), filter
now supports an extended syntax to what it had in SVG1.1. Its value is now none | <filter-value-list>
(see the spec), and a filter-value-list can be a list of URLs or filter functions.
The easiest thing would be to support lists of URLs first; librsvg effectively behaves as only supporting a single URL for filter
.