void ValidateRgn( CRgn* pRgn );
pRgn
Identifies a region that defines the area to be removed from the update region.
Validates the client area within the given region by removing the region from the current update region of the given window. If pRgn is NULL, the entire window is validated.
The given region must have been created previously by a region function. The region coordinates are assumed to be client coordinates.
::ValidateRgn