Metal: Fix undefined behavior of depth write Writing to an unbound depth attachment is undefined behavior in Metal. Fix this by emitting a function constant to guard depth buffer use in fragment shaders. Bug: angleproject:6865 Change-Id: Id7c10d0aeb349aacfe09c397bc292a71199ab50a Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3380304 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Kenneth Russell <kbr@chromium.org>