Chris Willman Music WriterAn animated river runs through Joni Mitchell’s video oeuvre now, as a 50-year-old song that has been gradually drafted into the holiday music canon, “River,” has just gotten its first music video. The clip made its debut with little time to spare in this “Blue” semicentennial year, premiering two days before Christmas.The video is stylized as a series of moving black-and-white water-colors, as it were, only opening up into actual color at the end.