Revert "set buffer_size at render time (in two places for forward and back compatibility) - refs https://github.com/mapnik/node-mapnik/issues/175 and https://github.com/mapbox/tilelive-vector/issues/5"

This reverts commit ef89afc151af485cdd316a3d9eceb2f9e5711632.
add todo wrt to https://github.com/mapnik/node-mapnik/issues/175