Update validation for GL_COMBINE. Bug 21278131 Change-Id: I9542d396cb7ea10b8ce2b5ef2050606e68cdffb9 Reviewed-on: https://swiftshader-review.googlesource.com/3233 Reviewed-by: Nicolas Capens <capn@google.com> Tested-by: Nicolas Capens <capn@google.com>