Despite his reputation as something of a hard-partying rock prankster (not many musicians play a guitar customized with an old beer bottle cap), Mac DeMarco, on record at least, has always been a hopeless romantic. And here, on his second album, the Canadian singer/songwriter effectively leans into loverman mode (just see “Let My Baby Stay”). But “Passing Out Pieces” is a particular marvel: Cast in cloudy synths and dark humor, it’s the sound of slacker rock’s clown prince getting serious.