Instead of storing a pointer to an ImageView, store a
pointer to an ImageViewHelper. The level and layer index
are all that's needed to pull the right image view from
the ImageViewHelper.

Will make it easier to store ImageView references in the
graph to track lifetime.

Bug: angleproject:2464
Change-Id: I773fa79a89ceefc99ff70e4eed4985f9f01fe26a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1857549
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>