“The Wizard of Oz” in 3-D IMAX? It could happen. IMAX co-chairman Greg Foster tells me there have been talks with Warner Bros., which controlls the Judy Garland classic. “But right now we’re running at capacity converting new movies to IMAX 3-D,” he adds. Next Wednesday, an IMAX version of “Harry Potter and the Order of the Phoenix,” with the climactic 20 minutes rendered in 3-D. I have an exclusive behind-the-scenes look at the process involved in today’s Post. Next up in 3-D IMAX: The entirety of “Beowulf,” Robert Zemeckis’s motion-picture animated epic, due in November.