Add other zoom levels to the GMap

[GoogleCodeExporter]

Adding higher zoom levels is straightforward because you should just need to 
divide the existing tile images by two (and multiply tile coordinates by two?).

Lower zoom levels are slightly more complicated because current tiles aren't 
necessarily at the origin of the zoomed-out tiles and the tile padding would 
need to be recalculated.  

Original issue reported on code.google.com by gbrammer on 28 Jul 2010 at 3:10