In some cases, when an unpack buffer is enabled, we would use the
incorrect layer offset, because we would think the pixel pointer
was NULL. Use a new parameter instead of the double "null" meaning
to fix this.

This fixes several tests in the dEQP texture specification group.
(EG: dEQP-GLES3.functional.texture.specification.teximage3d_pbo)

BUG=angle:899

Change-Id: If96bba6987a04653301efc348d32de372a237aa1
Reviewed-on: https://chromium-review.googlesource.com/244092Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>