Move parameter reading to a prototype constructor.

Bug 22652760

Change-Id: I317275cd2c15012da3a859735409af07ea9b2923
Reviewed-on: https://swiftshader-review.googlesource.com/4559
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>