Instead of having the translator output pre-rotation code in the vertex
stage based on a specialization constant, this change makes the SPIR-V
transformer perform pre-rotation of gl_Position on the last geometry
stage.

An alternative solution would be to generate pre-rotation code in the
translator in every geometry stage, each controlled by a separate
specialization constant.  This change avoids unnecessary modifications
to earlier stages.  The generated shaders are also smaller, as they
don't contain a mat2[8] pre-rotation constant matrix.  The SPIR-V
transformer knows the pre-rotation at transformation time, so it can
simply use swizzles to achieve the same results.

This also ties in with upcoming changes which move gl_Position.z
correction to the last geometry shader stage, which is trivially done
piggy-backing on the infrastructure in this change.

Bug: angleproject:5478
Change-Id: I9d5d9d19f3ccda665f5504368ce5ddfa5f383faf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2598584
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>