Previously the 12'th generic varying's w component would overwrite the
clipFlags member field.

Change-Id: I499979a2bbc653c0d95b35512a7a9e36667c2e4d
Reviewed-on: https://swiftshader-review.googlesource.com/5382Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>