Vulkan: Fix no location decoration vs framebuffer fetch The input attachment index is identical to the location qualifier. If there's only one output, GLSL is allowed to not specify the location qualifier, in which case it would implicitly be at location 0. Bug: angleproject:6947 Change-Id: Ib8e31ab524f6f4d4fe1d3e49386a374724da06a1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3566221 Reviewed-by: Cody Northrop <cnorthrop@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Lingfeng Yang <lfy@google.com>