make use of the TileStache projection to calculate tile co-ordinates
make use of the TileStache projection to calculate tile co-ordinates

The TileStache projection provider allows us to convert between tiles and projection co-ordinates and so we might as
well make use of it.

rjw57/foldbeam,rjw57/foldbeam,rjw57/foldbeam,rjw57/foldbeam