[Tilecache] dir & file naming algorithm

[Charlton Galvarino]

> You're ignoring the fact that your 'type=google' 'flips' things, so you
> have to
> subtract that y value from the maximum y value for the layer. In the case
> of
> your layer, it appears that you have 2^5 tiles in this zoom level in each
> direction, numbered 0-31 -- so the y is '31 - 13', or 18.

Brilliant, Chris.  That was the ticket.  Thanks for the help.

Charlton