In order to make the message handling available to windowless richedit
controls, the message handling must be in a function that can be
called from the ITextServices_TxSendMessage method.  This method will
never have a handle to a window to pass to RichEditWndProc_common in
order to get the editor with GetWindowLongPtrW, but passing the editor
will work (even if hWnd is NULL).