Function angle

jni/venus/region_operation.cpp:268–276

here are some useful links about hypot, I tried to use std::hypot(x, y), later changed to std::sqrt(a*a + b*b); http://stackoverflow.com/questions/3764978/why-hypot-function-is-so-slow also STL Bugs Fixed In Visual Studio 2012 718865 STL: hypot not hoisted into the std namespace by cmath https://blogs.msdn.microsoft.com/vcblog/2012/06/15/stl-bugs-fixed-in-visual-studio-2012/ */

	https://blogs.msdn.microsoft.com/vcblog/2012/06/15/stl-bugs-fixed-in-visual-studio-2012/
*/
static float angle(const Vec2f& v1, const Vec2f& v2)
{
	float numerator = v1[0]*v2[0] + v1[1]*v2[1];  // v1.dot(v2);
	float v1_length = std::sqrt(v1[0]*v1[0] + v1[1]*v1[1]);
	float v2_length = std::sqrt(v2[0]*v2[0] + v2[1]*v2[1]);
	float denominator = v1_length * v2_length;
//	assert(!isZero<T>(denominator));
	return std::acos(numerator/denominator);
}

static cv::Vec2f rotate(const cv::Vec2f& v, float angle)
{