code: public static TileGrid tileGridZoomDecrease(TileGrid tileGrid,
  			int zoomLevels) {
  		long minX = tileGridMinZoomDecrease(tileGrid.getMinX(), zoomLevels);
  		long maxX = tileGridMaxZoomDecrease(tileGrid.getMaxX(), zoomLevels);
  		long minY = tileGridMinZoomDecrease(tileGrid.getMinY(), zoomLevels);
  		long maxY = tileGridMaxZoomDecrease(tileGrid.getMaxY(), zoomLevels);
  		TileGrid newTileGrid = new TileGrid(minX, minY, maxX, maxY);
  		return newTileGrid;
  	}
docs: Get the tile grid starting from the tile grid and zooming out /
  decreasing the number of levels
  
  @param tileGrid
  current tile grid
  @param zoomLevels
  number of zoom levels to decrease by
  @return tile grid at new zoom level
  @since 2.0.1