Fix T79324: Crash when changing View Layer while VR session runs
Proper handling of View Layers for the VR session was never implemented. Now the View Layer of the VR session follows the window the session was started in. Note that if this window is closed, we fallback to another window. This is done to avoid the overhead it would take to maintain a separate depsgraph for the VR view. Instead we always share some already visible View Layer (and hence the depsgraph).