If the image needs to be redefined with mips, level 0 was copied to a
buffer which was then copied to the new image.  This change instead
stages the old image directly for copy to the new image.

Bug: angleproject:4551
Change-Id: I7625f140ddadde0a2b439c5e91c519ad49ae2fd7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2257264Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>