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.

aurora-main/include/aurora/aurora.h

@@ -180,6 +180,13 @@ typedef struct {
   uint64_t displayTimeNanos;
   // Optional; inactive when zero-initialised.
   AuroraCockpit cockpit;
+  // Immersive replay only: shows the race through a window rather than all
+  // around. Each eye keeps what it sees through the 2D layer's screen
+  // (aurora_set_stereo_hud_screen's rectangle, on which the 2D layer is then
+  // always placed), with premultiplied alpha 1 there, and is transparent black
+  // everywhere else, for the host's compositor to show its own background
+  // (the room, on a headset with passthrough) around it.
+  bool window;
 } AuroraStereoFrame;
 
 /**