Retrieves the update region into a region identified by pRgn.
Identifies the update region.
Specifies whether the background will be erased and nonclient areas of child windows will be drawn. If the value is FALSE, no drawing is done.
The coordinates of this region are relative to the upper-left corner (client coordinates).
The BeginPaint member function automatically validates the update region, so any call to GetUpdateRgn made immediately after a call to BeginPaint retrieves an empty update region.