Add state for GL_COMBINE.

Bug 21278131

Change-Id: I0590b67be5fad5690739b8ebaa9245cba4187b40
Reviewed-on: https://swiftshader-review.googlesource.com/3231
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>