Avoid double initialization of the upscaler singleton. The graphics initialization event might get called multiple times.

src/FSR3UnityPlugin.cpp

@@ -48,6 +48,12 @@ static void UNITY_INTERFACE_API OnGraphicsDeviceEvent(UnityGfxDeviceEventType ev
     {
     case kUnityGfxDeviceEventInitialize:
     {
+        if (s_Upscaler != nullptr)
+        {
+            // Avoid double initialization
+            break;
+        }
+
         s_RendererType = s_Graphics->GetRenderer();
         switch (s_RendererType)
         {