let the postprocessors use the mapnik::map object intellegently

so we needed some more information about the tile we were working with in the postprocessor (well at least to be able to do merging it was handy). so i've added sending the mapnik::map object along to the processing function of the izers. this fixes a bug in the zoom level config stuff that decides based on the config which izer to run on which requests. it was always passing in 0 for every request which means only the izers configured to run on zl 0 would have been run on any requests.
Since `scale` and `zoom` are inversely related, shouldn't `minscale = zoom_to_scale(maxzoom)`? And similar for `maxscale`?