Vulkan: Implement depth/stencil blit with flip of the viewport

- This makes all end2end tests pass successfully on Win/Nvidia
with viewport flipping enabled.

Bug: angleproject:2673
Change-Id: I031c0a0f5fb0aede58b59479e57c2c127bcb964c
Reviewed-on: https://chromium-review.googlesource.com/1133703
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Luc Ferron <lucferron@chromium.org>
diff --git a/src/libANGLE/renderer/renderer_utils.cpp b/src/libANGLE/renderer/renderer_utils.cpp
index 07a68a1..a0b287c 100644
--- a/src/libANGLE/renderer/renderer_utils.cpp
+++ b/src/libANGLE/renderer/renderer_utils.cpp
@@ -366,6 +366,7 @@
     }
 
     ColorWriteFunction colorWriteFunction = GetColorWriteFunction(formatType);
+    ASSERT(colorWriteFunction != nullptr);
 
     // Maximum size of any Color<T> type used.
     uint8_t temp[16];