Math: avoid dependency on Functions.h in DualQuaternion.h.

Is it still justifiable to do things like this or am I too paranoid
about compilation times now?

src/Magnum/Math/DualQuaternion.h

@@ -34,7 +34,6 @@
 #include "Magnum/Math/Dual.h"
 #include "Magnum/Math/Matrix4.h"
 #include "Magnum/Math/Quaternion.h"
-#include "Magnum/Math/Functions.h"
 
 namespace Magnum { namespace Math {
 
@@ -70,9 +69,8 @@ template<class T> inline DualQuaternion<T> sclerp(const DualQuaternion<T>& norma
 
     /* Avoid division by zero */
     const T cosHalfAngle = dotResult + normalizedA.real().scalar()*normalizedB.real().scalar();
-    if(std::abs(cosHalfAngle) >= T(1)) {
-        return DualQuaternion<T>{normalizedA.real(), Quaternion<T>{Math::lerp(normalizedA.dual().vector(), normalizedB.dual().vector(), t), T(0)}};
-    }
+    if(std::abs(cosHalfAngle) >= T(1))
+        return {normalizedA.real(), {Implementation::lerp(normalizedA.dual().vector(), normalizedB.dual().vector(), t), T(0)}};
 
     /* l + εm = q_A^**q_B, multiplying with -1 ensures shortest path when dot < 0 */
     const DualQuaternion<T> diff = normalizedA.quaternionConjugated()*(dotResult < T(0) ? -normalizedB : normalizedB);

src/Magnum/Math/Functions.h

@@ -437,10 +437,10 @@ The interpolation for vectors is done as in following, similarly for scalars: @f
 template<class T, class U> inline T lerp(const T& a, const T& b, U t);
 #else
 template<class T, class U> inline typename std::enable_if<!Implementation::IsBoolVector<U>::value, T>::type lerp(T a, T b, U t) {
-    return T((U(1) - t)*a + t*b);
+    return T(Implementation::lerp(a, b, t));
 }
 template<std::size_t size, class T, class U> inline typename std::enable_if<!Implementation::IsBoolVector<U>::value, Vector<size, T>>::type lerp(const Vector<size, T>& a, const Vector<size, T>& b, U t) {
-    return (U(1) - t)*a + t*b;
+    return Implementation::lerp(a, b, t);
 }
 #endif
 

src/Magnum/Math/Vector.h

@@ -50,6 +50,10 @@ namespace Magnum { namespace Math {
 
 namespace Implementation {
     template<std::size_t, class, class> struct VectorConverter;
+    /* Needed by DualQuaternion and Functions.h (to avoid dependency between them) */
+    template<class T> T lerp(const T& a, const T& b, U t) {
+        return T((U(1) - t)*a + t*b);
+    }
 }
 
 /** @relatesalso Vector