code: public void useCustomViewport(int x, int y, int width, int height) {
  		customViewport = true;
  		viewportX = x;
  		viewportY = y;
  		viewportWidth = width;
  		viewportHeight = height;
  	}
docs: Sets rendering to custom viewport with specified position and size
  <p>Note: you will be responsible for update of viewport via this method
  in case of any changes (on resize)