* Return the bucket which is offset from the specified dlon, dlat by
* the specified tile units in the X & Y direction.
* @param dlon starting lon in degrees
* @param dlat starting lat in degrees
* @param x number of bucket units to offset in x (lon) direction
* @param y number of bucket units to offset in y (lat) direction
* Return the bucket which is offset from the specified dlon, dlat by
* the specified tile units in the X & Y direction.
* @param dlon starting lon in degrees
* @param dlat starting lat in degrees
* @param x number of bucket units to offset in x (lon) direction
* @param y number of bucket units to offset in y (lat) direction