For a generation of black South African artists who came of age in the apartheid era, art and activism were intertwined; the liberation struggle was their life force. Now, a quarter of a century after Mandela became president, things are more complicated. Stephen Sackur speaks to John Kani, a giant of South African theatre. His career spans five decades of acting and writing. He’s been in Hollywood blockbusters, and is currently starring in his own West End play. What drives his artistic vision?