This function selects a region as the current clipping region for the specified device context.
The return value specifies the region's complexity.
NULLREGION indicates that the region is empty.
SIMPLEREGION indicates that the region is a single rectangle.
COMPLEXREGION indicates that the region is more than a single rectangle.
ERROR indicates that an error occurred; the current clipping region is unaffected.
To get extended error Information, call GetLastError.
Only a copy of the selected region is used. The region itself can be selected for any number of other device contexts or it can be deleted.
The SelectClipRgn function assumes that the coordinates for a region are specified in device units.
To remove a device-context's clipping region, specify a NULL region handle.