These patches fix painting in an application that draws outside of a CS_PARENTDC child client area in its WM_PAINT handler.
Comments and suggestions are welcome.
-- v2: win32u: Use parent rectangle for visible region calculations of a CS_PARENTDC child. win32u: Don't clip update region to the window client rectangle. win32u: GetUpdateRgn() should clip update region to the window client area. win32u: GetUpdateRect() should clip update rectangle to the window client area. win32u: Clip PAINTSTRUCT.rcPaint to the window client area. server: For a CS_PARENTDC child use parent for visible region calculations. server: If the being validated region covers whole window then validate everything.