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.
Has this option of scale in the constructor always existed, or is this a new feature and needs to be protected by an ifdef MAPNIK_VERSION?