code: public Envelope tileBbox(int tx, int ty, int zoomLevel) {
          Coordinate topLeft = tileTopLeft(tx, ty, zoomLevel);
          // upperLeft of tx+1,ty+1 == lowRight
          Coordinate lowerRight = tileTopLeft(tx + 1, ty + 1, zoomLevel);
          Envelope result = new Envelope(topLeft, lowerRight);
          return result;
      }
docs: Returns the EPSG:3857 bounding of the specified tile coordinate
  
  @param tx        The tile x coordinate
  @param ty        The tile y coordinate
  @param zoomLevel The tile zoom level
  @return the EPSG:3857 bounding box