Vulkan: Refactor RenderTargetVk ImageViews. 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>