func (s *Sprite) In(x, y int) bool {
	// Check the actual color (alpha) value at the specified position
	// so that the result of In becomes natural to users.
	//
	// Note that this is not a good manner to use At for logic
	// since color from At might include some errors on some machines.
	// As this is not so important logic, it's ok to use it so far.
	return s.image.At(x-s.x, y-s.y).(color.RGBA).A > 0
}
func (s *Sprite) In(x, y int) bool {
	// Check the actual color (alpha) value at the specified position
	// so that the result of In becomes natural to users.
	//
	// Note that this is not a good manner to use At for logic
	// since color from At might include some errors on some machines.
	// As this is not so important logic, it's ok to use it so far.
	return s.image.At(x-s.x, y-s.y).(color.RGBA).A > 0
}
// In returns true if (x, y) is in the sprite, and false otherwise.