Metal: Fix PBO readback failures with small row length Do not use blits when the row length is smaller than the source area width to avoid Metal validation failures. Bug: angleproject:354005999 Change-Id: Ifa0f29a0d1d2fb3275ea254957c24f36807e6f66 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5724790 Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com> Reviewed-by: Quyen Le <lehoangquyen@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org>