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.

Cherry-picked to the chromium/3538 branch cleanly.

Bug: chromium:880906
Change-Id: Iead2a8c57674e8962915902d6d5896f44fe8ca88
Reviewed-on: https://chromium-review.googlesource.com/1227033Reviewed-by: Jamie Madill <jmadill@chromium.org>