This adds a FramebufferVk cache. Cache signature is based on unique Serial
values that are assigned to ImageHelper objects backing all color and DS
rendertargets as well as level/layer values unique to the imageView.

Update the Serials and cache signature at FramebufferVk::syncState() time.
L0 cache is a currently active framebuffer.
L1 cache retrieves previously created framebuffer from new cache.
If neither of those hit, create new FramebufferVk and add to L1.

Bug: angleproject:4322
Change-Id: I3f585271798ddfb9e5f194020adca8cf8a6b19dd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2033869
Commit-Queue: Tobin Ehlis <tobine@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>