Jane Horwitz, Special to The Washington Post “Rock Dog,” an ungainly animated fable about a Tibetan mastiff who longs to be a guitar-shredding rocker, never really comes to heel. Intermittently diverting as it may be, the movie bears all the earmarks of a cobbled-together, made-by-committee product, poorly aimed at its tween-and-younger target audience in look, tone, music and story. Although scripted and computer-animated in the United States, “Rock Dog” is loosely based on a 2009 graphic novel, “Tibetan Rock Dog,” by Chinese rock star Zheng Jun.