When James Cameron’s “Titanic” gets its theatrical rerelease in 3-D next week, it will emerge into a very different world for movies than when it first came out. It can be hard to notice how much has changed since 1997 just by watching a contemporary blockbuster like “Transformers” or “Twilight.” But the shifts have been massive, and significant. The emergence of digital technology has given audiences more entertainment options than ever, while simultaneously opening up new ways for fans to find each other and discuss pieces of pop culture. As the Web provides ever-more information at an ever-quicker pace, new tools for making movies have allowed filmmakers to cut up and recombine images and sound at the furious pace our entertainment consumption now seems to require. And all of these changes are visible in a single piece of film marketing: the movie trailer.