code: function nextShallow(stream) {
      if (!stream.cursor) return false;
      var cursor = stream.cursor;
      cursor.x++;
      var bbox = stream.bboxes[cursor.z];
      if (cursor.x > bbox.maxX) {
          cursor.x = bbox.minX;
          cursor.y++;
      }
      if (cursor.y > bbox.maxY) {
          stream.cursor = false;
          return false;
      }
      return true;
  }
docs: Increment a tile cursor to the next position on the lowest zoom level.