This produces undefined pixel values in GL. We can also return
UNSUPPORTED if required and still be conformant. This fixes an
assert in the gfx-rs portability implementation.

Bug: angleproject:5601
Change-Id: Ifb84ad9b5e2365360a81cddba766b0bea81f2d51
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2668311
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>