Route VR lifecycle through platform services

src/app_vr.cpp

@@ -7,11 +7,6 @@
 #include "shape.h"
 #include "renderer_gl/opengl_capabilities.h"
 
-#ifdef _WIN32
-bool win32_vr_start();
-void win32_vr_stop();
-#endif
-
 namespace {
 
 void set_active_texture_unit(std::uint32_t unit_index)
@@ -104,18 +99,12 @@ Sphere controller_ray;
 
 bool App::vr_start()
 {
-#ifdef _WIN32
-    return win32_vr_start();
-#else
-    return false;
-#endif
+    return start_platform_vr_mode();
 }
 
 void App::vr_stop()
 {
-#ifdef _WIN32
-    win32_vr_stop();
-#endif
+    stop_platform_vr_mode();
 }
 
 void App::vr_draw_ui()