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

                                                      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.

See Also: INT 2fH 40xxH: Virtual Display Driver Functions
          INT 2fH: Multiplex Interrupt
          DOS Functions