三角形内，均匀生成点

算法来自于PBRT CH13

	std::default_random_engine eng(time(NULL));
	std::uniform_real_distribution<> urd(0., 1.);
	
	std::vector<osg::Vec3d> PTs2;
	for (int i = 0; i < 10000; i++)
	{
		double r1 = urd(eng);
		double r2 = urd(eng);
		double sqrt_r1 = std::sqrt(r1);
		
		double u = 1 - sqrt_r1;
		double v = r2 * sqrt_r1;
		double w = 1 - u - v;

		osg::Vec3d p = P1 * u + P2 * v + PT3 * w;
		PTs2.push_back(p);
	}