Allow not specifying a fragment output location.

Change-Id: I76c6e6efdff3161586f5d5422e97731969b9396c
Reviewed-on: https://swiftshader-review.googlesource.com/17028Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>