Fix nans in math::slerp, second attempt.

My earlier fix (3b373e5) was insufficient in some cases, this new
approach is cribbed from glMatrix.

libs/math/include/math/TQuatHelpers.h

@@ -245,17 +245,17 @@ public:
     friend inline
     QUATERNION<T> MATH_PURE slerp(const QUATERNION<T>& p, const QUATERNION<T>& q, T t) {
         // could also be computed as: pow(q * inverse(p), t) * p;
-        const T d = dot(p, q);
+        const T d = std::abs(dot(p, q));
+        static constexpr T value_eps = T(10) * std::numeric_limits<T>::epsilon();
+        // Prevent blowing up when slerping between two quaternions that are very near each other.
+        if ((T(1) - d) < value_eps) {
+            return lerp(p, q, t);
+        }
         const T npq = sqrt(dot(p, p) * dot(q, q));  // ||p|| * ||q||
-        const T a = std::acos(std::abs(d) / npq);
+        const T a = std::acos(d / npq);
         const T a0 = a * (1 - t);
         const T a1 = a * t;
         const T sina = sin(a);
-        // Prevent blowing up when slerping between two quaternions that are very near each other.
-        static constexpr T value_eps = T(10) * std::numeric_limits<T>::epsilon();
-        if (std::abs(sina) < value_eps) {
-            return lerp(p, q, t);
-        }
         const T isina = 1 / sina;
         const T s0 = std::sin(a0) * isina;
         const T s1 = std::sin(a1) * isina;