BlitGL: handle copyTexImage3D for the luma workaround BUG=angleproject:1492 Change-Id: Ica9a7c72f0e463850dd93f4a15d9b572fb10a1f6 Reviewed-on: https://chromium-review.googlesource.com/412054 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>