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).
This replaces header include guards with `#pragma once`.
A couple of include guards are not removed yet (e.g. `__RNA_TYPES_H__`),
because they are used in other places.
This patch has been generated by P1561 followed by `make format`.
Differential Revision: https://developer.blender.org/D8466
Splits up wm_xr.c into multiple files in their own folder:
source/blender/windowmanager/xr. So this matches how the message bus and
gizmo code have their own folder and files.
This allows better structuring and should make the code scale better.
I rather do this early on than to wait until we end up with a single,
huge file.
Also improves a bit how data is prepared and updated for drawing.
