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/tests/vr_config_tests.cpp

@@ -34,5 +34,18 @@ int main() {
     Require(Parse("[vr]\nsingle_pass_eyes = true\n").vrSinglePassEyes == true);
     Require(Parse("[vr]\nsingle_pass_eyes = false\n").vrSinglePassEyes == false);
     Require(!Parse("[vr]\n").vrSinglePassEyes.has_value());
+
+    // [vr] immersive_window and flat_screen: one race view in two keys, Flat
+    // Screen mode winning, so a file that predates the window reads as before.
+    using RuntimeConfigFile::VrRaceView;
+    using RuntimeConfigFile::VrRaceViewOf;
+    Require(Parse("[vr]\nimmersive_window = true\n").vrImmersiveWindow == true);
+    Require(!Parse("[vr]\n").vrImmersiveWindow.has_value());
+    Require(VrRaceViewOf(Parse("[vr]\n")) == VrRaceView::Immersive);
+    Require(VrRaceViewOf(Parse("[vr]\nflat_screen = false\n")) == VrRaceView::Immersive);
+    Require(VrRaceViewOf(Parse("[vr]\nflat_screen = true\n")) == VrRaceView::FlatScreen);
+    Require(VrRaceViewOf(Parse("[vr]\nimmersive_window = true\n")) == VrRaceView::ImmersiveWindow);
+    Require(VrRaceViewOf(Parse("[vr]\nflat_screen = true\nimmersive_window = true\n")) == VrRaceView::FlatScreen);
+    Require(VrRaceViewOf(Parse("[vr]\nflat_screen = false\nimmersive_window = false\n")) == VrRaceView::Immersive);
     return 0;
 }