Route VR lifecycle through platform services

src/platform_windows/windows_platform_services.cpp

@@ -21,6 +21,8 @@ void win32_renderdoc_frame_end();
 void win32_update_fps(int frames);
 void win32_update_stylus(float dt);
 void win32_save_window_state();
+bool win32_vr_start();
+void win32_vr_stop();
 
 namespace {
 
@@ -423,6 +425,16 @@ public:
         destroy_window();
     }
 
+    [[nodiscard]] bool start_vr_mode() override
+    {
+        return win32_vr_start();
+    }
+
+    void stop_vr_mode() override
+    {
+        win32_vr_stop();
+    }
+
     void pick_image(pp::platform::PickedPathCallback callback) override
     {
         const std::string path = open_file("Image Files (*.jpg, *.png)\0*.jpg;*.png");