In GimpTileBackendPlugin, return NULL when fetching z>0 tiles,
instead of simply ignoring the z coordinate, so that the mipmapped
tile is rendered locally.  Likewise, avoid storing z>0 tiles.

Note that this is suboptimal, since all the necessary level-0 tiles
need to be sent to the buffer as a result.  Ideally, we should
extend the wire protocol to handle mipmapped tiles.