Invalidates the entire client area of CWnd.
The bErase parameter specifies whether the background within the update area is to be erased when the update region is processed. If bErase is TRUE, the background is erased when the BeginPaint member function is called; if bErase is FALSE, the background remains unchanged. If bErase is TRUE for any part of the update region, the background in the entire region, not just in the given part, is erased.
Windows sends a WM_PAINT message whenever the CWnd update region is not empty and there are no other messages in the application queue for that window.