bug#23144: shrinking windows with gtk 3.20

[Paul Eggert]

As I understand it, the current version (3.20.4) of Gtk+ incorporates the
cleanup patch
<https://git.gnome.org/browse/gtk+/commit/?id=67ab00e01ec06d7b05a82c3d80b235ac6c4abfd2>
that causes the problem with Emacs 25 that Matthias noted in
<http://bugs.gnu.org/23144>. I haven't tested this, though.

As the bug was fixed in the Emacs master branch early last month
<http://git.savannah.gnu.org/cgit/emacs.git/commit/?id=afc9e6542aa6f2d0d7986db3d8c2cb654e107443>,
and nobody has seemed to complain, I'm becoming more inclined to cherry-pick the
fix into the emacs-25 branch. I'm by no means an expert in this area, but it
does seem to be a looming problem if we continue to do nothing.