SCALE option to produce retina tiles

Adds a `SCALE` option to `renderd.conf`. This way you can have retina (2x) tiles: just set `TILESIZE=512` + `SCALE=2`. This fork is currently producing tiles [here](http://osmz.ru/vretina.html), exactly with those options.

Also this PR fixes default buffer to be derived from tile size.
It is there since June 2010, that is, Mapnik 0.7.2 (and 2.0.0).