“National Gallery,” a meandering trip through the famed London art institution, has many sublime scenes — and some tedious ones, too. The documentary’s best moments are when no words are spoken, when the camera lingers on the paintings and the museum visitors gazing at them. The worst moments are when we visit the boardrooms and offices, where museum officials talk incessantly about mostly mundane matters that make our eyes glaze over. Director Frederick Wiseman has a great idea for a film here, but the subject matter (or the footage) doesn’t warrant a three-hour running time. The scenes begin to repeat themselves, and the film’s thin narrative premise gets exposed.