Retrieves a pointer to the DXSurface used as a background.
HRESULT retVal = object.GetBackground(ppSurface);
Address of a pointer to the interface through which to access the background surface.
If there is currently no background surface, this method returns S_OK, but ppSurface is set to NULL. When the method is implemented, it automatically calls AddRef on the interface. The client is responsible for calling Release on the interface when it is no longer needed.