Vulkan: Set gl_Layer to 0 if the framebuffer is not layered Bug: angleproject:372390039 Change-Id: I29067c9488e06f6dd2e90f207fecb843267fb77c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5949263 Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Yuxin Hu <yuxinhu@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>