For some reason, this wasn't implemented when texture image load and
store was implemented.

Bug: angleproject:4311
Change-Id: I24c508660ad7eb3aa944601175803751b54ce0ff
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2011324Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Ian Elliott <ianelliott@google.com>