Rasterizer discard is a feature that allows the graphics pipeline
to skip the fragment shader stage.

Implementing rasterizer discard in Vulkan is as easy as not binding
a fragment shader binary.

Tests for rasterizer discard live in
dEQP-GLES3.functional.rasterizer_discard.*.

Bug: angleproject:3214
Test: dEQP-GLES3.functional.rasterizer_discard.*
Change-Id: I0815df3b70f6f9cdc6c8c87ec4813bb629d8bd5f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1646692
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>