Fix bugs in 2D Array Texture support.

This fixes assertion failures and other errors, reproducible
by running the dEQP Texture Format tests.

BUG=angle:813

Change-Id: I3b97f89323f9656b45f617211fb4579a24013951
Reviewed-on: https://chromium-review.googlesource.com/229351
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>