Fix UtfGridComposite to work with latest TileStache

getTile line wasn't working at all anymore, fixed.
The UtfGridComposite object gets kept around, but was assuming it would only be used once, as such I've moved the member variables into local variables inside renderTile so we can be reused correctly.
Otherwise tiles would end up with the contents of other tiles and all sorts of badness.

I wrote the UtfGridComposite, so these changes should be okay :)