sort data for shapefile extracts

[ajashton]

Mapnik's painters algorithm renders features in the order they come from the datasource. It would be useful to generate shapefiles in an order that makes sense for this. I believe adding order by z_order asc to the queries will handle this for both the Imposm and osm2pgsql shapefiles.

[NelsonMinar]

this seems like a good idea if it's as simple as the order by

[migurski]

I can definitely do that—it won't hit all cases but it'll work for a lot.