We went to the Paul McCartney show at SAP Center on Wednesday night with the hottest new item in concert fashion: A clear, 12-inch-by-12-inch backpack. The three-hour concert by the former Beatle — a fantastic show, if that needed to be said — was the first major event at SAP Center since it rolled out the new clear-bag policy July 1, joining a growing list of stadiums and arenas that have done so since the NFL led the way in 2013. If I had a time machine, I’d go back a few years and dump every penny I could find into a company that makes clear backpacks. It would be like buying Apple stock in 1980. The new policy was not without its bumps. There were lines snaking around both sides of the downtown San Jose arena more than an hour before the show, but it went relatively fast and smoothly once things got moving. For those who didn’t know about the new policy, an outdoor bag check was available for $5. (Anyone going to the Queen + Adam Lambert show Sunday at SAP Center can consider themselves forewarned.) For us, the backpack took the place of my wife’s purse, so it held her wallet, keys, an empty water bottle, a phone and a sweatshirt. You can still bring in a clutch purse for p...