Vulkan: Fix FramebufferVk cache Migrate Serial from Image to ImageView. Imageviews are what are utimately used in FramebufferVk, so move the Serials into the ImageViewHelper class. Since that class also knows the level/layer of the imageView, we can revert to using a single Serial per ImageView instead of the AttachmentSerial that included the layer and level. ImageViewHelper caches Serials per layer/level combo. Bug: angleproject:4651 Change-Id: I3741d7d03523eada84295cb712c1cc1e6e3c3867 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2248203 Commit-Queue: Tobin Ehlis <tobine@google.com> Reviewed-by: Mohan Maiya <m.maiya@samsung.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>