prefix CSS URL-property with SVG-filename for Firefox

[flobacher]

Check if external stylesheet contains a styledef with an URL e.g.:

.className{
    fill: url(#bg-arrow--gradient-yellow-linear-gradient-1);
}

Since this is most likely from the injected svg make sure to not suffix the definition when injecting it, inject it only once for all browsers except firefox. Firefox only needs an absolute URL, so best is to use the svg-filename or the one of the spritesheet...