It's rare that big studio films, especially Marvel properties, let their security lax to such a degree that anyone can get video as clear, and of so crucial a scene, as this leaked set video from Alan Taylor's 'Thor: The Dark World.' In the video, Thor (Chris Hemsworth) does battle with Malekith the Accursed (Christopher Eccleston). It's not clear from the video that it is in fact Malekith, but t…