We wrap SVG data from icons within another SVG with extra styling
information. The wrapped SVG may contain characters that cannot be
part of a data: URL (https://fetch.spec.whatwg.org/#data-urls).
Librsvg 2.45 got more strict in its parsing of data: URLs; whereas
previously it ignored '#' characters in them, now it considers them to
be the start of a fragment identifier, which is not allowed in data:
URLs anyway.
To avoid unallowed characters, we now create a data: URL with a
base-64 encoded SVG.
Fixes https://gitlab.gnome.org/GNOME/gtk/issues/1471
