You don’t get to be a renowned producer of new plays and visionary artists, as Magic Theatre is, by cherry-picking scripts that magically emerge, fully formed, straight from the playwright’s mind. World premieres — which the Magic has a proud legacy of championing — are often the product of lengthy incubation periods, years of back-and-forth. With its Martha Heasley Cox Virgin Play Festival, the 50-year-old theater company lets audiences in on that creative process.