Vulkan: Cache ImageView serials on texture changes. This significantly reduces overhead when changing textures before draw calls in the Vulkan back-end. Bug: angleproject:6776 Change-Id: I24e2938c0122c3b23db4a90ec85b7d8f459cc063 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3583360 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yuxin Hu <yuxinhu@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>