“Oz the Great and Powerful,” a prequel to the iconic 1939 adaptation of L. Frank Baum’s “The Wizard of Oz,” is being framed as a huge gamble for Disney. Reviving that widely loved, fiercely protected property might draw huge audiences, or it might provoke an intense backlash. Judged on its own merits, which is admittedly difficult to do, “Oz” is a visually spectacular, predictable, yet entertaining movie.