Aliasing back_to_viewer -> back_turned.

Reason: Back_turned is already aliased with view_from_behind, so I see no reason to keep back_to_viewer separate.

(By the way, back is currently aliased to back_to_camera. Which seems synonymous with view_from_behind. Maybe they should all be lumped under the same tag? Or back_to_camera should be unaliased from back and aliased to view_from_behind instead.)