The depth read only or not should not affect VkFramebuffer creation.
RenderPasses with just depthstencil layout differences are considered
compatible. This CL moves this out of FramebufferDesc into
FramebufferVk.

Bug: b/168953278
Change-Id: I5bd05b262b7b3b0dc70f9fb8fc4a3db5e7082916
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2425032
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>