Math: Cleanup ScLERP implementation.

- Use explicit conversion to `T`
 - Use `std::` for `acos, cos, sin` to avoid use of double only functions
 - Do not mutate variables in math code to avoid confusion

Signed-off-by: Squareys <Squareys@googlemail.com>

src/Magnum/Math/DualQuaternion.h

@@ -66,20 +66,20 @@ template<class T> inline DualQuaternion<T> sclerp(const DualQuaternion<T>& from,
 
     const T dotResult = dot(from.real().vector(), to.real().vector());
 
-    /* Multiplying with -1.0f ensures shortest path when dot < 0. */
+    /* Multiplying with -1 ensures shortest path when dot < 0 */
     /* diff = \hat d = p^*q */
-    const DualQuaternion<T> diff = from.quaternionConjugated()*((dotResult < .0) ? -to : to);
+    const DualQuaternion<T> diff = from.quaternionConjugated()*((dotResult < T(0)) ? -to : to);
 
     /* angle = t \hat a_D */
-    const T angle = acos(diff.real().scalar())*t;
+    const T angle = std::acos(diff.real().scalar())*t;
     /* precompute sin/cos for manifold use */
-    const T sinAngle = sin(angle);
-    const T cosAngle = cos(angle);
+    const T sinAngle = std::sin(angle);
+    const T cosAngle = std::cos(angle);
 
     /* merely a shortcut */
     const Vector3<T>& m = diff.real().vector();
     /* invr = \frac 1 {|l_V|} */
-    const T invr = 1/m.length();
+    const T invr = m.lengthInverted();
     /* direction = \hat {\boldsymbol n} = \hat l_V \frac 1 {|l_V|} */
     const Vector3<T> direction = m*invr;
 
@@ -88,14 +88,14 @@ template<class T> inline DualQuaternion<T> sclerp(const DualQuaternion<T>& from,
     const Vector3<T> v = direction*sinAngle;
 
     /* pitch = \frac {\hat a_D} 2 = m_S \frac 1 {|l_V|} */
-    T pitch = -diff.dual().scalar()*invr;
+    const T pitch = -diff.dual().scalar()*invr;
     /* moment = \hat {\boldsymbol n}_D */
     const Vector3<T> moment = (diff.dual().vector() - (direction*(pitch*diff.real().scalar())))*invr;
-    pitch *= t;
+    const T pitchT = pitch*t;
     /* Vector of dual part of q_{ScLERP} */
-    const Vector3<T> v2 = moment*sinAngle + direction*(pitch*cosAngle);
+    const Vector3<T> v2 = moment*sinAngle + direction*(pitchT*cosAngle);
 
-    return from*DualQuaternion<T>{Quaternion<T>{v, cosAngle}, Quaternion<T>{v2, -pitch*sinAngle}};
+    return from*DualQuaternion<T>{Quaternion<T>{v, cosAngle}, Quaternion<T>{v2, -pitchT*sinAngle}};
 }
 
 /**