Instead of having the translator output code to transform gl_Position.z
to Vulkan clip space in the vertex stage, this change makes the SPIR-V
transformer perform this operation on the last geometry stage.

An alternative solution would be to generate this transformation in the
translator in every geometry stage, each controlled by a separate
specialization constant.  This change avoids unnecessary modifications
to earlier stages.  Additionally, the transformer is already modifying
gl_Position.xy for pre-rotation, so the addition of a small
transformation of gl_Position.z in the same spot is rather trivial.

Bug: angleproject:5479
Change-Id: Id971179ba47b206204bfdaf3b2b295ef97dd5117
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2598585
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>