“Derek,” a fragmentary portrait of the British filmmaker, painter, set designer and writer Derek Jarman, is a cinematic scrapbook of the life and times of an iconoclast, aesthete and provocateur who died of AIDS in 1994. Assembled by the director Isaac Julien, the fragments are clustered around a poetic epistle, “Letter to an Angel,” written by Mr. Jarman’s friend Tilda Swinton and published in