RoundEven implementation

Implementation for the roundEven glsl intrinsic function.
All dEQP roundEven tests pass.

Change-Id: I8b6f3704f03eea32b08a6c2dc318ecb99a01957d
Reviewed-on: https://swiftshader-review.googlesource.com/3382
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>