bug#42406: Mouse-wheel scrolling can be flickering

[martin rudalics]

> That won't work without also changing redisplay_internal to consider
> such a list.  It currently knows about just 2 alternatives: either
> consider only the selected window on the selected frame, or all
> windows on all frames -- and only the second option considers
> non-window aspects (such as the frame title or menu bar) of the
> selected frame.

The part mostly affected would be

  if (consider_all_windows_p)
    {
      FOR_EACH_FRAME (tail, frame)
        XFRAME (frame)->updated_p = false;

because windows_or_buffers_changed should probably become either a list
too or redisplay would simply scan all windows and act when it finds a
window whose w->update_mode_line or w->changed flag is true.

martin