LONDON (Reuters) – "Avatar" director James Cameron and Russian conductor Valery Gergiev will beam the world's first live 3D ballet broadcast to 50 countries from Russia's Mariinsky theatre in June. Cinema audiences across Europe, the United States, South America, Asia, Australia and elsewhere will be able to watch Ekaterina Kondaurova dance the role of Odette/Odile in Tchaikovsky's "Swan Lake" at the 19th century opera house on June 6, production company Omniverse Vision said in a statement. …