Roger Waters didn’t set out to be one of the greatest songwriters of all time. Pink Floyd was destined to be the kind of psychedelic band that took rock and roll to new heights whenever they played, but when Syd Barrett found himself out of step with the rest of the band, Waters didn’t really have a choice over what his destiny was anymore. If the band was going to stay together, he was going to need to become a new songwriter, and that meant le…