“The Late Works: Clyfford Still in Maryland” is a heartfelt effort by the late artist’s namesake museum to seal the deal on his lifelong legacy, to demonstrate, with significant evidence, that Still was a preeminent visual trailblazer from the moment he picked up a brush right through his final days. This later body of work, from 1961 to 1980, is not the stuff that Still’s vast reputation is made from.