Mapnik renderer fallback to OSM when network available about monav HOT 1 OPEN

Seems like a nice idea. My thoughts about this:

 * The downloaded tiles could be saved in the MapnikRenderer format, slowly filling the files. Loading tiles from a gradually filled file would be slower, though, as adjacent tiles are not necessarily adjacent in the file any more. The format would still require the index file to be initialized properly. Once a tile is written to the file it cannot be changed anymore.
 * Tiles from lower zoom levels could be upscaled until a replacement has been downloaded
 * It must be possible to switch off downloading tiles ( e.g. to avoid roaming coasts on a mobile device )
 * One problem I foresee is that accumulating tiles slowly could lead to inconsistencies in the displayed map.
 * A preprocessor plugin without Mapnik as a dependency should still be available as Mapnik is not easy to get / compile ( many dependencies ).