Vulkan: Fix missing input attachment usage on MSRTT When combined with advanced blend (or framebuffer fetch for that matter), MSRTT attachments could be used as input attachments. This change fixes the missing usage bit. Bug: angleproject:7739 Change-Id: I2190e4a6e534e120357dd68189fb9de07b6c54fa Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3936444 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yuxin Hu <yuxinhu@google.com> Reviewed-by: Amirali Abdolrashidi <abdolrashidi@google.com>