Metal: Fix bytesPerRow computation for compressed formats The value represents the row size in bytes, not pixels. Bug: angleproject:7209 Change-Id: Ib7fade31f13ffd3ae78c5df313120c8e0201936c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3588416 Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Kenneth Russell <kbr@chromium.org>