code: public void inset(float dx, float dy) {
          left += dx;
          top -= dy;
          right -= dx;
          bottom += dy;
      }
docs: Inset the viewport by (dx,dy). If dx is positive, then the sides are moved inwards, making the viewport narrower.
  If dx is negative, then the sides are moved outwards, making the viewport wider. The same holds true for dy and
  the top and bottom.
  
  @param dx The amount to add(subtract) from the viewport's left(right)
  @param dy The amount to add(subtract) from the viewport's top(bottom)