Validates the client area within the given region by removing the region from the current update region of the window.
The given region must have been created previously by a region function. The region coordinates are assumed to be client coordinates.
The BeginPaint member function automatically validates the entire client area. Neither the ValidateRect nor the ValidateRgn member function should be called if a portion of the update region must be validated before the next WM_PAINT message is generated.