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

source/creator/CMakeLists.txt

@@ -111,6 +111,10 @@ if(WITH_FREESTYLE)
   add_definitions(-DWITH_FREESTYLE)
 endif()
 
+if(WITH_XR_OPENXR)
+  add_definitions(-DWITH_XR_OPENXR)
+endif()
+
 # Setup the exe sources and buildinfo
 set(SRC
   creator.c
@@ -856,6 +860,8 @@ elseif(WIN32)
       ${CMAKE_SOURCE_DIR}/release/windows/batch/blender_debug_gpu_glitchworkaround.cmd
       ${CMAKE_SOURCE_DIR}/release/windows/batch/blender_debug_log.cmd
       ${CMAKE_SOURCE_DIR}/release/windows/batch/blender_factory_startup.cmd
+      ${CMAKE_SOURCE_DIR}/release/windows/batch/blender_oculus.cmd
+      ${CMAKE_SOURCE_DIR}/release/windows/batch/oculus.json
     DESTINATION "."
   )
 

source/creator/creator_args.c

@@ -603,6 +603,10 @@ static int arg_handle_print_help(int UNUSED(argc), const char **UNUSED(argv), vo
   BLI_argsPrintArgDoc(ba, "--debug-gpu-shaders");
   BLI_argsPrintArgDoc(ba, "--debug-gpu-force-workarounds");
   BLI_argsPrintArgDoc(ba, "--debug-wm");
+#  ifdef WITH_XR_OPENXR
+  BLI_argsPrintArgDoc(ba, "--debug-xr");
+  BLI_argsPrintArgDoc(ba, "--debug-xr-time");
+#  endif
   BLI_argsPrintArgDoc(ba, "--debug-all");
   BLI_argsPrintArgDoc(ba, "--debug-io");
 
@@ -940,6 +944,16 @@ static const char arg_handle_debug_mode_generic_set_doc_wm[] =
     "\n\t"
     "Enable debug messages for the window manager, shows all operators in search, shows "
     "keymap errors.";
+#  ifdef WITH_XR_OPENXR
+static const char arg_handle_debug_mode_generic_set_doc_xr[] =
+    "\n\t"
+    "Enable debug messages for virtual reality contexts.\n"
+    "\tEnables the OpenXR API validation layer, (OpenXR) debug messages and general information "
+    "prints.";
+static const char arg_handle_debug_mode_generic_set_doc_xr_time[] =
+    "\n\t"
+    "Enable debug messages for virtual reality frame rendering times.";
+#  endif
 static const char arg_handle_debug_mode_generic_set_doc_jobs[] =
     "\n\t"
     "Enable time profiling for background jobs.";
@@ -2091,6 +2105,16 @@ void main_args_setup(bContext *C, bArgs *ba)
               (void *)G_DEBUG_HANDLERS);
   BLI_argsAdd(
       ba, 1, NULL, "--debug-wm", CB_EX(arg_handle_debug_mode_generic_set, wm), (void *)G_DEBUG_WM);
+#  ifdef WITH_XR_OPENXR
+  BLI_argsAdd(
+      ba, 1, NULL, "--debug-xr", CB_EX(arg_handle_debug_mode_generic_set, xr), (void *)G_DEBUG_XR);
+  BLI_argsAdd(ba,
+              1,
+              NULL,
+              "--debug-xr-time",
+              CB_EX(arg_handle_debug_mode_generic_set, xr_time),
+              (void *)G_DEBUG_XR_TIME);
+#  endif
   BLI_argsAdd(ba,
               1,
               NULL,