Implemented modf as trunc + sub
Passes all related dEQP tests

Change-Id: I43656c51a670d235153e5fac390a8db311b14f8d
Reviewed-on: https://swiftshader-review.googlesource.com/5280Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>