Use the correct internal format for swaps.

Change-Id: If0aedc86a1f6492eb8174744f9ca6c3eea24e729
Reviewed-on: https://swiftshader-review.googlesource.com/3090
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>