[FL] · Filter linkage.  Using an *xlink:href* to inherit these filter nodes from base.svg#shadow
       would be better than copying them, but apparently Chrome (59) does not supported that.

[NIL]  No interdocument links.  Else the referent elements fail to render (Firefox 52, Chrome 59).
       The workaround employed here: manually copy the element declarations to whatever documents
       need them.