The late philanthropist Paul Mellon gave $218 million and 900 pictures to the National Gallery of Art. “‘We thought of bringing them all together, but we couldn’t,’ said director Earl A. ‘Rusty’ Powell III. ‘There’d have been nothing left in the galleries.'” The museum, which is mounting a Boudin show instead, is one of 19 institutions around the world marking Mellon’s centennial this year.