Fix regression with librsvg

Instead of using xmlCreateIOParserCtxt, librsvg pushes its own
xmlParserInput on top of a memory push parser. This incorrect use of
the API confuses several parser checks and, since 2.9.5, completely
breaks documents with internal subsets. Work around the problem with
internal subsets.

Thanks to Petr Sumbera for the report:

    https://mail.gnome.org/archives/xml/2017-September/msg00011.html

Also see

    https://bugzilla.gnome.org/show_bug.cgi?id=787895