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
Only the volume object is exposed in the user interface. It is based on OpenVDB
internally. Drawing and rendering code will follow in another commit.
https://wiki.blender.org/wiki/Source/Objects/Volume
https://wiki.blender.org/wiki/Reference/Release_Notes/2.83/Volumes
Hair and PointCloud object types are hidden behind a WITH_NEW_OBJECT_TYPES
build option. These are unfinished, and included only to make it easier to
cooperate on development in the future and avoid tricky merges.
https://wiki.blender.org/wiki/Source/Objects/New_Object_Types
Ref T73201, T68981
Differential Revision: https://developer.blender.org/D6945