Various tips for Mapnik

Use of render_to_file() from Mapnik 0.7.0

(20:17:58) springmeyer: dmentre: also note that in Mapnik 0.7.0 you can do `render_to_file(m,'map.pdf')`
(20:18:13) springmeyer: which allows saving to a cairo format while bypassing pycairo
(20:18:28) springmeyer: not too useful for you guys since you are passing pycairo objects, but might be faster
(20:18:51) springmeyer: if you are writing to a file just a simple (non cairo composed) png map