VChangeNumberOfOverlays - changes the number of overlays that can be used
VChangeNumberOfOverlays changes the number of overlays and updates the
COLOR.COM file. If the number of overlays is the same as before,
VChangeNumberOfOverlays does nothing and returns. VChangeNumberOfOverlays
should be called after VCreateWindows and VCreateColormap have been called.
0 - work successfully.
3 - write error.
num_ovl - specifies the new number of overlays. At present the maximum
number of overlays is one.
Any existing overlay graphics may be destroyed.
If num_ovl is not valid, or VCreateColormap is not called earlier,
VChangeNumberOfOverlays prints an error message to the standard error stream,
produces a core dump file and exits from the current process.