Dump assets and "initial" data before rendering starts

Now does preprocessing off of main() instead of in the dispatcher
initializer.

overviewer.py

@@ -380,6 +380,12 @@ dir but you forgot to put quotes around the directory, since it contains spaces.
         tset = tileset.TileSet(rset, assetMrg, tex, tileSetOpts, tileset_dir)
         tilesets.append(tset)
 
+    # Do tileset preprocessing here, before we start dispatching jobs
+    for ts in tilesets:
+        ts.do_preprocessing()
+
+    # Output initial static data and configuration
+    assetMrg.initialize(tilesets)
    
     # multiprocessing dispatcher
     if config['processes'] == 1: