Add two more storage backends ro_http_proxy and ro_composite

Both are read only backends, i.e. they can't be used in renderd, but only in mod_tile

ro_http_proxy is a proxy storage backend that retrieves tiles from a remote http tile server

ro_composite is a storage backend that composits two tiles before passing them up.

Although ro_composite can composite tiles from any two storage backends, conceptually it only
works well, if at least one set of tiles are static non updating tiles, as the updating logic
does not allow for requesting rendering updates for both tilesets
 


git-svn-id: http://svn.openstreetmap.org/applications/utils/mod_tile@29447 b9d5c4c9-76e1-0310-9c85-f3177eceb1e4