Chris Willman Senior Music Writer and Chief Music Critic In the history of album cover art, only two parties have become arguably almost as famous as some of the bands they shot or designed for: Hipgnosis, in the 1970s, and Anton Corbijn, from the mid-‘80s forward. Those two forces come together with a new documentary, “Squaring the Circle (The Story of Hipgnosis),” directed by Corbijn, about the era of artwork that entranced him, like most music fans, as a youth. Forty years after Hipgnosis ceased to be a going concern as we knew it, the company’s covers for Pink Floyd, Led Zeppelin and countless others continue to be collected in coffee-table books. And when it comes to the stories behind the sleeves, rock fans still just can’t get enough… to quote a band that Corbijn came to be associated with a little bit later.