Make stoq work with new weasyprint

New weasyprint uses cairocffi, but the when rendering, all we have is a
pygtk cairo.Context, and cairocffi does not handle that well. We need to
convert it to the apropriate object, as decribed here:

https://github.com/Kozea/WeasyPrint/issues/130

and here:

http://pythonhosted.org/cairocffi/cffi_api.html#converting-pycairo-wrappers-to-cairocffi

Fixes: http://bugs.async.com.br/show_bug.cgi?id=5606
Change-Id: I910e147702feaec645912659bb4fadbf0803b0bc