This allows us to override it in the incomplete texture init. Any
back-end that used incomplete textures was vulnerable to a bug where
the unpack buffer would be used to initialize the incomplete texture.

Bug: chromium:880906
Change-Id: Ica558e4a4d81de9212f0bc6619ccd812a048ad45
Reviewed-on: https://chromium-review.googlesource.com/1214207Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>