code: func (s *Sprite) MoveBy(x, y int) {
  	w, h := s.image.Size()
  
  	s.x += x
  	s.y += y
  	if s.x < 0 {
  		s.x = 0
  	}
  	if s.x > screenWidth-w {
  		s.x = screenWidth - w
  	}
  	if s.y < 0 {
  		s.y = 0
  	}
  	if s.y > screenHeight-h {
  		s.y = screenHeight - h
  	}
  }
docs: // MoveBy moves the sprite by (x, y).