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, 51 insertions, 2 deletions

diff --git a/source/blender/draw/CMakeLists.txt b/source/blender/draw/CMakeLists.txt
index a1213be4be0..81f2214b402 100644
--- a/source/blender/draw/CMakeLists.txt
+++ b/source/blender/draw/CMakeLists.txt
@@ -390,4 +390,8 @@ if(WITH_FREESTYLE)
   add_definitions(-DWITH_FREESTYLE)
 endif()
 
+if(WITH_XR_OPENXR)
+  add_definitions(-DWITH_XR_OPENXR)
+endif()
+
 blender_add_lib(bf_draw "${SRC}" "${INC}" "${INC_SYS}" "${LIB}")
diff --git a/source/blender/draw/DRW_engine.h b/source/blender/draw/DRW_engine.h
index b4c23d5e57c..7ecf9df275d 100644
--- a/source/blender/draw/DRW_engine.h
+++ b/source/blender/draw/DRW_engine.h
@@ -93,6 +93,7 @@ void DRW_draw_render_loop_offscreen(struct Depsgraph *depsgraph,
                                     struct RenderEngineType *engine_type,
                                     struct ARegion *region,
                                     struct View3D *v3d,
+                                    const bool is_image_render,
                                     const bool draw_background,
                                     const bool do_color_management,
                                     struct GPUOffScreen *ofs,
@@ -139,6 +140,14 @@ void DRW_opengl_context_destroy(void);
 void DRW_opengl_context_enable(void);
 void DRW_opengl_context_disable(void);
 
+#ifdef WITH_XR_OPENXR
+/* XXX see comment on DRW_xr_opengl_context_get() */
+void *DRW_xr_opengl_context_get(void);
+void *DRW_xr_gpu_context_get(void);
+void DRW_xr_drawing_begin(void);
+void DRW_xr_drawing_end(void);
+#endif
+
 /* For garbage collection */
 void DRW_cache_free_old_batches(struct Main *bmain);
 
diff --git a/source/blender/draw/intern/draw_manager.c b/source/blender/draw/intern/draw_manager.c
index 71e2d8d9b57..618922d8544 100644
--- a/source/blender/draw/intern/draw_manager.c
+++ b/source/blender/draw/intern/draw_manager.c
@@ -562,7 +562,7 @@ static void drw_viewport_var_init(void)
     DRW_view_camtexco_set(DST.view_default, rv3d->viewcamtexcofac);
 
     if (DST.draw_ctx.sh_cfg == GPU_SHADER_CFG_CLIPPED) {
-      int plane_len = (rv3d->viewlock & RV3D_BOXCLIP) ? 4 : 6;
+      int plane_len = (RV3D_LOCK_FLAGS(rv3d) & RV3D_BOXCLIP) ? 4 : 6;
       DRW_view_clip_planes_set(DST.view_default, rv3d->clip, plane_len);
     }
 
@@ -1554,6 +1554,7 @@ void DRW_draw_render_loop_offscreen(struct Depsgraph *depsgraph,
                                     RenderEngineType *engine_type,
                                     ARegion *region,
                                     View3D *v3d,
+                                    const bool is_image_render,
                                     const bool draw_background,
                                     const bool do_color_management,
                                     GPUOffScreen *ofs,
@@ -1569,7 +1570,7 @@ void DRW_draw_render_loop_offscreen(struct Depsgraph *depsgraph,
 
   /* Reset before using it. */
   drw_state_prepare_clean_for_draw(&DST);
-  DST.options.is_image_render = true;
+  DST.options.is_image_render = is_image_render;
   DST.options.do_color_management = do_color_management;
   DST.options.draw_background = draw_background;
   DRW_draw_render_loop_ex(depsgraph, engine_type, region, v3d, render_viewport, NULL);
@@ -2829,4 +2830,39 @@ void DRW_gpu_render_context_disable(void *UNUSED(re_gpu_context))
   GPU_context_active_set(NULL);
 }
 
+#ifdef WITH_XR_OPENXR
+
+/* XXX
+ * There should really be no such getter, but for VR we currently can't easily avoid it. OpenXR
+ * needs some low level info for the OpenGL context that will be used for submitting the
+ * final framebuffer. VR could in theory create its own context, but that would mean we have to
+ * switch to it just to submit the final frame, which has notable performance impact.
+ *
+ * We could "inject" a context through DRW_opengl_render_context_enable(), but that would have to
+ * work from the main thread, which is tricky to get working too. The preferable solution would be
+ * using a separate thread for VR drawing where a single context can stay active. */
+void *DRW_xr_opengl_context_get(void)
+{
+  return DST.gl_context;
+}
+
+/* XXX See comment on DRW_xr_opengl_context_get(). */
+void *DRW_xr_gpu_context_get(void)
+{
+  return DST.gpu_context;
+}
+
+/* XXX See comment on DRW_xr_opengl_context_get(). */
+void DRW_xr_drawing_begin(void)
+{
+  BLI_ticket_mutex_lock(DST.gl_context_mutex);
+}
+
+/* XXX See comment on DRW_xr_opengl_context_get(). */
+void DRW_xr_drawing_end(void)
+{
+  BLI_ticket_mutex_unlock(DST.gl_context_mutex);
+}
+
+#endif
 /** \} */