“Avatar” will be back on 3-D Imax screens in August, and it will be longer than ever. Imax and 20th Century Fox on Friday said they planned to re-release James Cameron’s science fiction epic for a two-week run beginning Aug. 27. The film, which already had a running time of 162 minutes, will include eight minutes of new material. Fox executives had been mulling a re-release ever since “Avatar,” which came out last Dec. 18, got nudged off of Imax screens in March by Walt Disney’s “Alice in Wonderland.” The blockbuster has sold about $2.7 billion in tickets worldwide.