bug#38181: Actual height of mode-line not taken into account

[Eli Zaretskii]

> Date: Wed, 06 May 2020 12:27:52 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: jonas@bernoul.li, 38181@debbugs.gnu.org
> 
> I simply don't yet understand why updating the frame decorations that have 
> nothing to do with text layout needs init_iterator to run.  What am I 
> missing?  Redisplay doesn't in general redraw the whole frame, it only 
> redraws windows.

Answering my own question: because the clear_under_internal_border
method, which redraws the internal border, is called from
redisplay_internal.