Compatibility: Win 3.x
Expects: AX    4001H
Returns: none
Info: VM-aware applications can intercept this fn.  The VDD calls it
when it is switching the application into the background.

After being switched into the background, the application
continues running, but will be frozen if it attempts to access
video memory (and the screen is in a mode in which background
access is not supported by the VDD--see INT 2fH 4000H).

- -

INT 2fH 4001H: Notify Background Switch