Compatibility: Win 3.x
Expects: AX    4000H
Returns: AL    video support: 00H no support provided
01H no modes virtualized in background
02H only text modes virtualized in background
03H only text and single-plane graphics modes
04H only text, single plane and VGA gfx modes
ffH all supported video modes virtualized
Info: This allows a VM-aware application to gain more control over how
the screen is saved and restored by the VDD (Virtual Device
driver).  This is typically called when the VM-aware app is
initializing.  After calling:

  The VDD saves the current settings of the video registers.  The
VDD will use these settings before calling INT 2fH 4005H and
INT 2fH 4006H (save and restore video register).

  The VDD will use INT 2fH 4005H and INT 2fH 4006H when it needs
to save and restore video register settings.  The idea is that
you can provide your own handler for these fns.

  The VDD will no longer save the contents of the virtual display
across screen switches.  It will be your responsibility to
intercept INT 2fH 4002H and refresh your own video mode and
screen when you are switched to the foreground.

Use INT 2fH 4007H (disable VM-assisted video save/restore) to
return to allowing the VDD to have full responsibilities.

- -

INT 2fH 4000H: Enable VM-Assisted Video Save/Restore