Fix CopyBufferSubData validation. We were checking readTarget twice, instead of both read and write. BUG=angle:800 Change-Id: I4096a9c4e0fdec9c77bfc25250923c188d2b43db Reviewed-on: https://chromium-review.googlesource.com/224102 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>