Split Apple platform target and move platform state ownership

src/platform_windows/windows_platform_services.h

@@ -4,9 +4,12 @@
 
 #include <future>
 
+struct VrSessionSnapshot;
+
 namespace pp::platform::windows {
 
 [[nodiscard]] PlatformServices& platform_services();
+[[nodiscard]] VrSessionSnapshot read_platform_vr_session_snapshot() noexcept;
 void enqueue_main_thread_task(std::packaged_task<void()> task);
 void drain_main_thread_tasks();