Vulkan: Flip viewport on y for the backbuffer only

- Hide the implementation behind a feature flag, currently
disabled permanently as I'm working on fixing the different
failures.
- SimpleOperationTest.* passing

Bug: angleproject:2673
Change-Id: Ic86520c3cc478d62bebbaeaf4c6b33c439a67b0f
Reviewed-on: https://chromium-review.googlesource.com/1119089
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/Framebuffer.cpp b/src/libANGLE/Framebuffer.cpp
index 9bd2d02..60494c0 100644
--- a/src/libANGLE/Framebuffer.cpp
+++ b/src/libANGLE/Framebuffer.cpp
@@ -1522,6 +1522,11 @@
     return mImpl->blit(context, sourceArea, destArea, blitMask, filter);
 }
 
+bool Framebuffer::isDefault() const
+{
+    return id() == 0;
+}
+
 int Framebuffer::getSamples(const Context *context)
 {
     return (isComplete(context) ? getCachedSamples(context) : 0);