Vulkan: Fix varyings reserved for xfb At some point in the past, the ANGLEXfbPosition varying was generated unconditionally when the xfb extension is available. The logic that reserved one varying for this was not updated and fails to account for it in recent drivers. Bug: angleproject:362545033 Change-Id: I65fb58cd912b96b2774a7dfbffa4aa656f694e81 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5826761 Auto-Submit: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Charlie Lao <cclao@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>