message: Fix nans in math::slerp, second attempt.
  My earlier fix (3b373e5) was insufficient in some cases, this new
  approach is cribbed from glMatrix.
diff: @@ -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;
  