Fix 2D tex array buffer unpacking. 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/244092 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Zhenyao Mo <zmo@chromium.org>