RGBColor類定義

這個類主要是顏色操作，操作詳細原理如下圖：

類聲明：

class RGBColor {
public:
	RGBColor();
	~RGBColor();
	RGBColor(ldouble a);
	RGBColor(ldouble red, ldouble green, ldouble blue);
	RGBColor(const RGBColor& c);
	RGBColor operator+(const RGBColor& c) const;
	RGBColor& operator+=(const RGBColor& c);
	RGBColor operator*(const ldouble a) const;
	RGBColor& operator*=(const ldouble a);
	RGBColor operator/(const ldouble f) const;
	RGBColor& operator/=(const ldouble f);
	RGBColor operator*(const RGBColor& c) const;
	RGBColor& operator=(const RGBColor& c);
	bool operator==(const RGBColor& c) const;
	bool operator!=(const RGBColor& c) const;
	RGBColor powc(ldouble p) const;
	ldouble average() const;
	friend std::ostream& operator<<(std::ostream& os, const RGBColor& c);
	friend std::istream& operator>>(std::istream& is, RGBColor& c);
	ldouble r, g, b;
};