wkhtmltopdf isn't using a fixed-width font for CSS font-family:monospace.
Nor is it using one for <PRE> <CODE> or <TT> elements.
Add in some Courier fonts for it to use - note that Courier 10 Pitch is
installed on Ubuntu by default. Note these fonts need to be installed on
the system that generates the pdf documentation.
Previously the htmldoc stylesheet was kept in place and the SWIG
stylesheet was prepended to it inline in SWIGDocumentation.html.
Now the SWIG stylesheet has been amended with most of the htmldoc
stylesheet changes and completely replaced after htmldoc is run.
