This was very wrong for 3D textures.

BUG=angleproject:1384

Change-Id: I7f042449e30e1e909778c0524d1ce99d20ddfd65
Reviewed-on: https://chromium-review.googlesource.com/348063Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>