Implement alpha testing.

Change-Id: I0fe062ea826b73b2bdf28ad3ed6e72e51c576840
Reviewed-on: https://swiftshader-review.googlesource.com/3635
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>