Use wxWindow::GetClientAreaOrigin() instead of MSW functions.

Use wxWindow::GetClientAreaOrigin() instead of MSW functions.

::AdjustWindowRectEx() doesn't seem to work correctly for wxPopupWindow and
still offsets the coordinates by the title bar height even if these windows
don't have WS_CAPTION style. Rather than try to work around this, simply use
wxWindow method instead of the Windows function as shape wxRegion offset.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69457 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775