message: timer: add add get_update_time()
  Used to get time unhandled by check_update_time(), which can be used for
  frame interpolation in the drawing stage.
diff: @@ -221,6 +221,16 @@ pub fn check_update_time(ctx: &mut Context, target_fps: u32) -> bool {
  }
  }
  +/// Returns the total time not consumed by `check_update_time()`
  +/// (the remainder or unhandled time).
  +///
  +/// The intention is for it to be called in your `draw()` callback
  +/// to interpolate phyisics states for smooth rendering.
  +/// (see <http://gafferongames.com/game-physics/fix-your-timestep/>)
  +pub fn get_update_time(ctx: &mut Context) -> time::Duration {
  + ctx.timer_context.residual_update_dt
  +}
  +
  /// Pauses the current thread for the target duration.
  /// Just calls `std::thread::sleep()` so it's as accurate
  /// as that is (which is usually not very).
  