Metal: skip copying via parentTexture on iOS simulator. iOS simulator has bug in [MTLTexture parentRelativeSlice] so don't use it to upload data to the texture. Bug: angleproject:343734719 Change-Id: I78cf0066c1120c04f9b62c30e280ad866c2096d7 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5584042 Reviewed-by: Kenneth Russell <kbr@chromium.org> Auto-Submit: Quyen Le <lehoangquyen@chromium.org> Commit-Queue: Kenneth Russell <kbr@chromium.org> Commit-Queue: Quyen Le <lehoangquyen@chromium.org>