* gegl/buffer/gegl-tile-storage.h: store the max z value seen instead
of whether we've seen != 0.
* gegl/buffer/gegl-tile.c: (_gegl_tile_void_pyramid): bail out at the
appropriate z-level for the underlying storage.
* gegl/buffer/gegl-tile-handler-zoom.c: (get_tile): update z value
seen.

svn path=/trunk/; revision=2744