This will allow us to call setImage internally even if there's an
unpack buffer bound. Useful when implementing robust resource init with
copyTexImage in some edge cases.

Bug: angleproject:4517
Change-Id: I7e8e9536ab9222c620e572f7b6c20b08fa29d646
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2133088
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>