“Writing the Future: Basquiat and the Hip-Hop Generation,” a brilliant exhibition at Boston’s Museum of Fine Arts, reveals the centrality of visual art to hip-hop’s thrilling beginnings. The show prompts fresh consideration of the origins of hip-hop and the “post-graffiti” movement, which saw the street artists who had transformed New York’s urban landscape adapt their work for display in high-end galleries, as well as in music videos and fashion.