Only for 2D array textures.

Bug: angleproject:3189
Change-Id: I9958ab60ffa464aa63f5cb321cbbe14b378f3647
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1709109
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>