(#1471): base64-encode included SVGs to avoid mis-escaped characters

(#1471): base64-encode included SVGs to avoid mis-escaped characters

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