This is done via inserting a statement at the end of the "main"
of the vertex shader which scales and offsets gl_Position.z.

Relevant sections of the Vulkan spec are 23.4. Coordinate
Transformations and OpenGL spec "2.12.1 Controlling the Viewport".

Bug: angleproject:2406
Change-Id: Ibf5403b3ad06a5114a7de3be0a406d696cbb7dea
Reviewed-on: https://chromium-review.googlesource.com/964733Reviewed-by: Geoff Lang <geofflang@chromium.org>