Leonardo da Vinci by Walter Isaacson Simon & Schuster, 2017, 624 pages, $49.99 _______________________________ Who has $450 million to spend on one painting? That’s what an anonymous bidder paid at Christie’s for perhaps the last Leonardo da Vinci painting out of captivity in November last year. Called Salvator Mundi, it had languished in a Swiss bank vault for years before a magnificent restoration job by a leading New York restorer. It was the highest price ever paid for a painting at auction. The leading experts on Leonardo have come down in favour of declaring it genuine, so it becomes one…