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>
3 files changed