Fix light position transformation.

Change-Id: Ia12819357406c6ee98f0af2909d793cb62bfa62f
Reviewed-on: https://swiftshader-review.googlesource.com/3510
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>