RSC transfer presents a crowd-pleasing mix of metatheatrical comedy and music with, ultimately, a touch of melancholy Don Quixote and his paunchy sidekick long ago escaped the pages of Miguel de Cervantes' novel. The image of the sad-faced knight on his bony nag Rocinante with his companion Sancho Panza atop his donkey are familiar in film, opera, paintings and everything from kitchen tiles to cartoons and furnishing…