Fragment output fix

Fragment outputs weren't given a proper index, so a
fragment output array has been added to fix this.

Change-Id: Id9be21f60cc23528bf1af078a8b3ca2df28e7ee4
Reviewed-on: https://swiftshader-review.googlesource.com/4295
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>