XPjoyX()
  Return the X coordinate of the Joystick
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  Function:    XPjoyX()

               This function will return the current X position
               of the stick. When positioned at the complete left,
               this should return 0 or values close to that, at
               the right it should return some maximum value (depending
               on the speed of the machine), and in the middle it should
               return about half of right's value. Your program should
               thus first initialise some machine dependant values. Once
               understood how it works, it is easy to figure out the rest.

  Syntax:      XPjoyX([nJoyNr]) --> nX

  Arguments:   JoyStick #1 if none is supplied, otherwise 0 indicates #1,
               1 indicates joystick #2.

  Returns:     A numerical, generally between 0 and . 255, machine
               dependant.

  Usage:       * Assuming JoyMiddle is some initialised variable ..
               if XPjoyX() < JoyMiddleX
                    MoveRight()
               endif