message: Add safe_rsqrt()
diff: @@ -1289,6 +1289,16 @@ template <typename Value> ENOKI_INLINE Value safe_sqrt(const Value &a) {
  return result;
  }
  +template <typename Value> ENOKI_INLINE Value safe_rsqrt(const Value &a) {
  + Value result = rsqrt(max(a, 0));
  +
  + if constexpr (is_diff_array_v<Value>) {
  + if (grad_enabled(a))
  + result = replace_grad(result, rsqrt(max(a, Epsilon<Value>)));
  + }
  +
  + return result;
  +}
  template <typename Value> ENOKI_INLINE Value safe_cbrt(const Value &a) {
  Value result = cbrt(max(a, 0));
  