Instead of refreshing sampler serials every time the SamplerVk or
TextureVk has a state change we can give a VkSampler a unique serial.
The serial is unique to this VkSampler and repeated state changes
will fetch the same Serial from the SamplerCache. This allows for
more cache hits.

We store the the new Serial together with the VkSampler in a
SamplerHelper class and store references to a SamplerHelper in
SamplerVk and TextureVk instead of the VkSampler directly.

In a follow-up change we will improve image view caching by also
improving how we store serials for ImageViews.

Bug: angleproject:4911
Change-Id: I9168c2700e383bca796cca925b38cfd30132d982
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2333988
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>