Fix setting the source format before using it.

Change-Id: I47b82f0e10e572943c31df521b1b74492ba6318e
Reviewed-on: https://swiftshader-review.googlesource.com/3081
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>