[URP] Fixing unstable 271_AlphaPassthrough_CameraStacking_RT test

* Updates a reference image for 271_AlphaPassthrough_CameraStacking_RT
* Raises the threshold a little bit

Author: ellioman

Tests/SRPTests/Projects/UniversalGraphicsTest_PostPro/Assets/Scenes/271_AlphaPassthrough_CameraStacking_RT.unity

@@ -329,7 +329,7 @@ MonoBehaviour:
     PerPixelGammaThreshold: 0
     PerPixelAlphaThreshold: 0
     RMSEThreshold: 0
-    AverageCorrectnessThreshold: 0.001208
+    AverageCorrectnessThreshold: 0.001211
     IncorrectPixelsThreshold: 0
     UseHDR: 0
     UseBackBuffer: 1