Stops playback.
If the virtual pen driver is in playback mode, but not actually playing back pen packets, then the VPEND_PENPLAYSTOP message takes the virtual pen driver out of playback mode. If the virtual pen driver is playing back pen packets when the VPEND_PENPLAYSTOP message is received, the driver will both stop playing pen packets and go out of playback mode.
In the sample virtual pen driver, the VPEND_PENPLAYSTOP message handler does its job by setting the pointer to the array of pen playback packets to NULL and turning off both the global flag that indicates the virtual pen driver is in playback mode and the global flag that indicates the virtual pen driver is playing back packets.