Vulkan does not have alpha test. Apps are expected to use discard in the
shader instead.

Change-Id: Idc1d8d2a74f5728cf70c653a5139a0ca1674ca37
Reviewed-on: https://swiftshader-review.googlesource.com/c/25211Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>