Move parameter reading to routine constructor.

Bug 22652760

Change-Id: If2b41c4d3b24f4b8a9d9dd805a9ab3ce724b7726
Reviewed-on: https://swiftshader-review.googlesource.com/4553
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>