This year’s River to River fest kicked off in a pretty monumental way on Sunday, with the annual Bang on a Can Marathon. For 12 straight hours, 150 avant-garde musicians and experimental composers filled the World Financial Center’s Winter Garden with everything from post-rock to drone, pipa, piano, and electronic tunes; the bill included Tortoise, plus a world-premiere piece by the Bang on a Can All-Stars, piano virtuoso Ryuichi Sakamoto, and legendary jazz bassist Henry Grimes.