Metal: prevent padding in state cache's structs. Fixed: angleproject:411845677 Change-Id: I8a9635634e66f86c132205f75825a916481f7ad0 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6468239 Commit-Queue: Quyen Le <lehoangquyen@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org>