RegisterBGIdriver function

  DECLARATION:  RegisterBGIdriver(Driver : Pointer) : Integer

      PURPOSE:  Registers a user-loaded or linked-in BGI driver with the
                graphics system.

         UNIT:  Graph

  RESULT TYPE:  Integer

      REMARKS:  With this, a user may load a driver file and "register"
                it by passing its memory location to RegisterBGIdriver.
                When that driver is used by InitGraph, the registered driver
                will be used instead of being loading from disk by the Graph
                unit.

                If an error occurs, the returned value is less than zero,
                otherwise the internal driver number is returned.

 RESTRICTIONS:  The driver must be registered before the call to
                InitGraph.  Calling RegisterBGIdriver after graphics have
                been activated will return a value of -11 (grError).