[OSM-dev] Fix for slowdown in python mapnik rendering

[Jon Burgess]

I've just fixed a nasty little bug which was causing the python mapnik
rendering scripts to slow down over time. I've put the fix into
generate_tiles.py and all_tiles/render_all.py in SVN.

If you're interested in the details, doing an explicit os.waitpid() call
leaks an entry in the _active list used by subprocess.py to track the
list of child processes.

	Jon