Fixing issue with MatrixAffine2::rotateCopy methods

Before that, the following would not compile due to:
No matching member function for call to 'rotateCopy'

ci::MatrixAffine2f m1;
auto m2 = m1.rotateCopy(M_PI * 0.5f);
auto m3 = m1.rotateCopy(M_PI * 0.5f, ci::Vec2f(100, 200));