Fixes the generation of 3D texture mipmaps.

BUG=angle:915

Change-Id: Icc0c91099f60713d511ebe1a2248a21b63efddaa
Reviewed-on: https://chromium-review.googlesource.com/244720Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Gregoire Payen de La Garanderie <Gregory.Payen@imgtec.com>