Blender Git Loki

Fix export stereo images from Image Editor when the image was a render

This fix is a worka around (I duplicate the ImBuf) to prevent locking
due to both buffers being related to the same element (LOCK_VIEWER).

That's an issue exclusive of rendered images (it wasn't happening for
MultiView EXRs).

To render/animate the scene directly to stereo output is not possible at
the moment. Only saving from Image Editor has it fully supported.