Use atan2() in Quaternion axis-and-angle

It's more numerically robust than acos() - we can't hit a range error
if we get strange rounding effects - and we've got the value of the
sin of the angle already (it's the length of the x, y, z part), so
might as well use it. As length is necessarily positive, atan2() will
give an upper-half-plane resolution, matching what acos() gave us.

This incidentally means that you no longer need to normalize() your
quaternion before you ask for axis and angle.

Task-number: QTBUG-114313
Change-Id: If3fa2b371c72991f1f8f151f78ef7f9180aa87cf
Reviewed-by: Matthias Rauter <matthias.rauter@qt.io>
Reviewed-by: Jøger Hansegård <joger.hansegard@qt.io>
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/gui/math3d/qquaternion.cpp

@@ -386,7 +386,7 @@ void QQuaternion::getAxisAndAngle(float *x, float *y, float *z, float *angle) co
             *y = yp / length;
             *z = zp / length;
         }
-        *angle = qRadiansToDegrees(2.0f * std::acos(wp));
+        *angle = qRadiansToDegrees(2.0f * std::atan2(length, wp));
     } else {
         // angle is 0 (mod 2*pi), so any axis will fit
         *x = *y = *z = *angle = 0.0f;