bug#7348: 23.2.50; Emacs crashes on fast window resize with scrollbars on under OSX

[martin rudalics]

> The call to do_pending_window_change at line 11397 in xdisp.c
> (emacs-23 branch) seems to change selected_window because of the
> following call chain, but the variable `w' in redisplay_internal still
> points to the old selected window.
>
>   redisplay_internal
>     -> do_pending_window_change
>       -> change_frame_size
>         -> change_frame_size_1
>           -> set_window_height
>             -> size_window
>               -> delete_window (/* Delete WINDOW if it's too small.  */)

That's bad.  So basing redisplay_internal entirely on

  struct window *w = XWINDOW (selected_window);

is inherently broken.  But simply reassigning

   w = XWINDOW (selected_window);

after every do_pending_window_change call is hairy since it changes the
selected window under our feet, so any things done for the window that
was selected before the call would probably have to be redone for the
now selected window.  OTOH going back to retry after every call that
might have changed the selected window could get us into an infinite
loop.  (BTW, do we really need up all three do_pending_window_change
calls in redisplay_internal?)

martin