VR: Initial Virtual Reality support - Milestone 1, Scene Inspection

NOTE: While most of the milestone 1 goals are there, a few smaller features and
improvements are still to be done.

Big picture of this milestone: Initial, OpenXR-based virtual reality support
for users and foundation for advanced use cases.
Maniphest Task: https://developer.blender.org/T71347
The tasks contains more information about this milestone.

To be clear: This is not a feature rich VR implementation, it's focused on the
initial scene inspection use case. We intentionally focused on that, further
features like controller support are part of the next milestone.

- How to use?
Instructions on how to use this are here:
https://wiki.blender.org/wiki/User:Severin/GSoC-2019/How_to_Test
These will be updated and moved to a more official place (likely the manual) soon.

Currently Windows Mixed Reality and Oculus devices are usable. Valve/HTC
headsets don't support the OpenXR standard yet and hence, do not work with this
implementation.

---------------

This is the C-side implementation of the features added for initial VR
support as per milestone 1. A "VR Scene Inspection" Add-on will be
committed separately, to expose the VR functionality in the UI. It also
adds some further features for milestone 1, namely a landmarking system
(stored view locations in the VR space)

Main additions/features:
* Support for rendering viewports to an HMD, with good performance.
* Option to sync the VR view perspective with a fully interactive,
  regular 3D View (VR-Mirror).
* Option to disable positional tracking. Keeps the current position (calculated
  based on the VR eye center pose) when enabled while a VR session is running.
* Some regular viewport settings for the VR view
* RNA/Python-API to query and set VR session state information.
* WM-XR: Layer tying Ghost-XR to the Blender specific APIs/data
* wmSurface API: drawable, non-window container (manages Ghost-OpenGL and GPU
  context)
* DNA/RNA for management of VR session settings
* `--debug-xr` and `--debug-xr-time` commandline options
* Utility batch & config file for using the Oculus runtime on Windows.
* Most VR data is runtime only. The exception is user settings which are saved
  to files (`XrSessionSettings`).
* VR support can be disabled through the `WITH_XR_OPENXR` compiler flag.

For architecture and code documentation, see
https://wiki.blender.org/wiki/Source/Interface/XR.

---------------

A few thank you's:
* A huge shoutout to Ray Molenkamp for his help during the project - it would
  have not been that successful without him!
* Sebastian Koenig and Simeon Conzendorf for testing and feedback!
* The reviewers, especially Brecht Van Lommel!
* Dalai Felinto for pushing and managing me to get this done ;)
* The OpenXR working group for providing an open standard. I think we're the
  first bigger application to adopt OpenXR. Congratulations to them and
  ourselves :)

This project started as a Google Summer of Code 2019 project - "Core Support of
Virtual Reality Headsets through OpenXR" (see
https://wiki.blender.org/wiki/User:Severin/GSoC-2019/).
Some further information, including ideas for further improvements can be found
in the final GSoC report:
https://wiki.blender.org/wiki/User:Severin/GSoC-2019/Final_Report

Differential Revisions: D6193, D7098

Reviewed by: Brecht Van Lommel, Jeroen Bakker

3 files changed, 10 insertions, 2 deletions

diff --git a/source/blender/blenkernel/BKE_global.h b/source/blender/blenkernel/BKE_global.h
index fe050075bae..f6cae6d8a9c 100644
--- a/source/blender/blenkernel/BKE_global.h
+++ b/source/blender/blenkernel/BKE_global.h
@@ -153,6 +153,8 @@ enum {
   G_DEBUG_IO = (1 << 17),                    /* IO Debugging (for Collada, ...)*/
   G_DEBUG_GPU_SHADERS = (1 << 18),           /* GLSL shaders */
   G_DEBUG_GPU_FORCE_WORKAROUNDS = (1 << 19), /* force gpu workarounds bypassing detections. */
+  G_DEBUG_XR = (1 << 20),                    /* XR/OpenXR messages */
+  G_DEBUG_XR_TIME = (1 << 21),               /* XR/OpenXR timing messages */
 
   G_DEBUG_GHOST = (1 << 20), /* Debug GHOST module. */
 };
diff --git a/source/blender/blenkernel/CMakeLists.txt b/source/blender/blenkernel/CMakeLists.txt
index 923108240dd..047901b4c81 100644
--- a/source/blender/blenkernel/CMakeLists.txt
+++ b/source/blender/blenkernel/CMakeLists.txt
@@ -667,6 +667,10 @@ if(WITH_TBB)
   )
 endif()
 
+if(WITH_XR_OPENXR)
+  add_definitions(-DWITH_XR_OPENXR)
+endif()
+
 # # Warnings as errors, this is too strict!
 # if(MSVC)
 #    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /WX")
diff --git a/source/blender/blenkernel/intern/object.c b/source/blender/blenkernel/intern/object.c
index d7247518f9e..fa3284c18d6 100644
--- a/source/blender/blenkernel/intern/object.c
+++ b/source/blender/blenkernel/intern/object.c
@@ -2680,7 +2680,8 @@ void BKE_object_workob_calc_parent(Depsgraph *depsgraph, Scene *scene, Object *o
 }
 
 /**
- * Applies the global transformation \a mat to the \a ob using a relative parent space if supplied.
+ * Applies the global transformation \a mat to the \a ob using a relative parent space if
+ * supplied.
  *
  * \param mat: the global transformation mat that the object should be set object to.
  * \param parent: the parent space in which this object will be set relative to
@@ -3183,7 +3184,8 @@ typedef struct ObTfmBack {
   float obmat[4][4];
   /** inverse result of parent, so that object doesn't 'stick' to parent. */
   float parentinv[4][4];
-  /** inverse result of constraints. doesn't include effect of parent or object local transform. */
+  /** inverse result of constraints. doesn't include effect of parent or object local transform.
+   */
   float constinv[4][4];
   /** inverse matrix of 'obmat' for during render, temporally: ipokeys of transform. */
   float imat[4][4];