Fix: GPU Tests on Windows

Use RAII for `GHOST_WindowClass`.

For the main Blender executable, only one `GHOST_System` is ever
created. But when running tests, this can happen multiple times (eg,
GPUOpenGLTest and GPUOpenGLWorkaroundsTest).
This results in `GHOST_SystemWin32` trying to register
`GHOST_WindowClass` multiple times, so `RegisterClassW` fails with
`ERROR_CLASS_ALREADY_EXISTS`.

Pull Request: https://projects.blender.org/blender/blender/pulls/144231

intern/ghost/intern/GHOST_System.cc

@@ -293,6 +293,9 @@ GHOST_TSuccess GHOST_System::init()
 
 GHOST_TSuccess GHOST_System::exit()
 {
+  /** WARNING: exit() may run more than once, since it may need to be called from a derived class
+   * destructor. Take it into account when modifying this function. */
+
   delete m_windowManager;
   m_windowManager = nullptr;
 

intern/ghost/intern/GHOST_SystemWin32.cc

@@ -195,6 +195,10 @@ GHOST_SystemWin32::~GHOST_SystemWin32()
   if (isStartedFromCommandPrompt()) {
     setConsoleWindowState(GHOST_kConsoleWindowStateShow);
   }
+
+  /* We must call exit from here, since by the time ~GHOST_System calls it, the GHOST_SystemWin32
+   * override is no longer reachable.   */
+  exit();
 }
 
 uint64_t GHOST_SystemWin32::performanceCounterToMillis(__int64 perf_ticks) const
@@ -661,7 +665,10 @@ GHOST_TSuccess GHOST_SystemWin32::init()
 
 GHOST_TSuccess GHOST_SystemWin32::exit()
 {
-  return GHOST_System::exit();
+  GHOST_TSuccess success = GHOST_System::exit();
+  /* All windows created with the specified class must be destroyed before unregistering it. */
+  ::UnregisterClassW(L"GHOST_WindowClass", ::GetModuleHandle(0));
+  return success;
 }
 
 GHOST_TKey GHOST_SystemWin32::hardKey(RAWINPUT const &raw, bool *r_key_down)