feat: added immersive window support for VR race views

- Introduced a new configuration option for immersive window mode in runtime_config.h.
- Updated the parsing and setting functions to handle the immersive window state.
- Modified the OpenXR backend to support rendering with the immersive window, blending the race view with the surrounding environment.
- Enhanced the settings overlay to allow users to select between immersive, immersive window, and flat screen race views.
- Implemented GPU rendering logic for the immersive window mask, ensuring correct visual output in various rendering paths.
- Added tests to validate the immersive window functionality and its interaction with existing race view settings.

runtime/include/vr/openxr_backend.h

@@ -132,9 +132,15 @@ struct OpenXRPresentation {
     bool quad_anchored = false;
     XrPosef quad_pose{{0.0f, 0.0f, 0.0f, 1.0f}, {0.0f, 0.0f, 0.0f}};
 
-    // Show the room through the headset's cameras around the virtual screen
-    // (OpenXRPassthrough). Taken when the presentation is handed to the backend,
-    // which starts or pauses the view then; a backend without one ignores it.
+    // Used only by ImmersiveProjection: Aurora left each eye transparent outside
+    // the race's 2D-layer screen (AuroraStereoFrame::window), so the projection
+    // layer is blended by its alpha over whatever is under it.
+    bool immersive_window = false;
+
+    // Show the room through the headset's cameras around the virtual screen or
+    // the immersive window (OpenXRPassthrough). Taken when the presentation is
+    // handed to the backend, which starts or pauses the view then; a backend
+    // without one ignores it.
     bool passthrough = false;
 
     OpenXRPanelLayer panel;

runtime/include/vr/openxr_integration.h

@@ -61,11 +61,19 @@ void OpenXRRequestRecenter() noexcept;
 void OpenXRSetLeanBackDegrees(float degrees) noexcept;
 
 // Shows the room through the headset's cameras around the menu screen and every
-// other virtual screen, never during an immersive race. Only the standalone
-// (Quest) backend offers it; elsewhere this changes nothing. Callable from any
-// thread; applied on the XR pacing thread's next frame.
+// other virtual screen, and around the immersive window, never during a fully
+// immersive race. Only the standalone (Quest) backend offers it; elsewhere this
+// changes nothing. Callable from any thread; applied on the XR pacing thread's
+// next frame.
 void OpenXRSetPassthrough(bool enabled) noexcept;
 
+// The immersive window: an immersive race keeps its stereo view but is seen
+// only through the screen its 2D layer sits on, with the room (or, without
+// passthrough, black) around it. Callable from any thread; applied to the next
+// published frame. Flat Screen mode, which keeps races off the immersive path
+// altogether, makes it moot.
+void OpenXRSetImmersiveWindow(bool enabled) noexcept;
+
 // Live scene interpolation at the headset's own display deadlines.
 // 0 = Off, 1 = Auto, otherwise 72/90/120 as a rendering-rate ceiling.
 void OpenXRSetFrameInterpolationFps(uint32_t target) noexcept;