One might be skeptical of Lorin Maazel’s new opera “1984,” getting its premiere this week at the Royal Opera House, writes Rupert Christiansen. But “stir the mixture, and the result is a cleverly concocted piece of operatic fast-food, stuffed with musical additives and devoid of substantial nurtritional value, but quite engrossing and enjoyable. The audience was clearly absorbed throughout and the reception was generally enthusiastic.”