It has never been easy for women in the music industry. Once upon a time the evidence was largely anecdotal. Now it’s being recorded for posterity, frame by frame. Recent documentaries about Britney Spears and Demi Lovato exposed the trauma inflicted on post-millennial pop stars. Two new portraits of Anna Mae Bullock and Marianne Elliott-Said, better known as Tina Turner and Poly Styrene from punk group X-Ray Spex, ponder the price paid by their forebears.