intersperse updates and frames more precisely

For https://github.com/PistonDevelopers/piston/issues/522

Please inspect closely; I changed some subtle things, like when we check time and where variables are set, so I may have had some unintended side effects.
I think this should be `self.last_update += self.dt_update_time_in_ns;` to not delay the time it takes to update application.