Instead of storing a dictionary of serials to specific image views
we now store a single 32-bit serial combined with subresource info.
The serials combined with a subresource info (level/layer) gives a
unique identifier for each ImageView in the ImageViewHelper for the
descriptor set cache and the Framebuffer cache.

Also moves ImageView serial allocation to initialization and release.
This means we no longer need to use "getAssign" methods and instead
we use a few init methods to ensure the serials stay allocated.

Bug: angleproject:4911
Change-Id: Ia6af76ae16b3ff5d4a83974bde05cc704064b079
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2333395
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>