[Tilesathome] Blank tile handling

[Rob Reid]

I think a recent patch, probably 9702 
<http://trac.openstreetmap.org/changeset/9702> may have gone a bit too 
far with blank tile handling.

I recently rendered the following tileset and all blank tiles appear to 
be showing up as sea including those that should be blank land

http://tah.openstreetmap.org/Browse/details/tile/12/3497/2135/
You need to zoom to z14 to see the problem.

Getting rid of the 0 bytes tiles is great but don't we still need the 
emptysea/emptyland tiles to indicate what type of blank tiles the tile 
in question and all subsequent zoom levels are?

I have a copy of the tileset I uploaded if you want it but its a pretty 
quick tileset to render.

Cheers

rcr