SEOUL — The couple saw brushes and paint cans in front of a paint-splattered canvas at a gallery in a Seoul shopping mall.... So they added a few brush strokes, assuming it was a participatory mural. Not quite: The painting was a finished work by an American artist whose abstract aesthetic riffs on street art.... And on social media, South Koreans are debating what the vandalism illustrates about art, authorship and authenticity. The artwork is displayed with paint cans, brushes and shoes that