This is a reland of ff38106d

Original change's description:
> Vulkan: Use specialization constant for halfRenderAreaWidth
>
> halfRenderArea is used by fragment shader to adjust gl_FragCoord for
> rotation and yflip compensation. This CL bakes halfRenderAreaWidth into
> shader via specialization constant, thus allow compiler to consolidate
> the calculation into one MAD instruction.
>
> Bug: b/173800146
> Change-Id: Id66301278e3389e2582369b695825e632bccecee
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2551541
> Commit-Queue: Charlie Lao <cclao@google.com>
> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
> Reviewed-by: Tim Van Patten <timvp@google.com>
> Reviewed-by: Jamie Madill <jmadill@chromium.org>

Bug: b/173800146
Change-Id: Ifc36086f4c5d6a44fb5456aa6e31bc8a783bba71
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2579648
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>