cssrewrite breaks SVG Data URI about webassets

[barbossa]

I confirm the issue. Someone has a solution?

[02JanDal]

Also ran into this, solved it by subclassing CSSRewrite and overriding rewrite_url to just returning the matched data if it seems to be a data-url (which doesn't make sense to rewrite anyway):

def rewrite_url(self, m):
    url = m.groups()[1]
    if url.startswith('data:') or url.startswith('"data:') or url.startswith('\'data:'):
        return m.group(0)
    else:
        return super(FixedCSSRewrite, self).rewrite_url(m)

I can put this into a PR if this is an acceptable solution.