Metal: Clear depth pbuffers to 1 for robust init Robust resource initialization would init depth pbuffers as 0.0 instead of 1.0. This would differ from the FBO initialization, where the uninitialized depth buffers would be initialized to 1.0. Bug: angleproject:348199042 Change-Id: I0f251e85d6e08ce992fe323b257a0906d237f5b1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5642763 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Kimmo Kinnunen <kkinnunen@apple.com> Auto-Submit: Kimmo Kinnunen <kkinnunen@apple.com> Reviewed-by: Quyen Le <lehoangquyen@chromium.org>