Font in mode line

[Francesco Mazzoli]

Hi,

I'm trying to set "8x13" (one of the fonts shipped with X) to be the font in 
my mode line.

If I start emacs using `emacs -Q', and then execute

    (set-face-attribute 'mode-line nil :font "8x13")

The font is rendered wrongly: <http://i.imgur.com/QUHaWnj.png>.  Note that 
the buffer id appears differently because it follows the `mode-line-buffer-
id' face, if I set that to use 8x13 too that is rendered wrongly as well.

Weirdly, if I set "8x13" to be the default font, it renders fine (both in the 
buffer and in the mode line): <http://i.imgur.com/o9QcxLr.png>.

Am I doing something wrong?  I'm running `GNU Emacs 24.3.1 (x86_64-pc-linux-
gnu, GTK+ Version 3.8.4) of 2013-09-27 on trouble, modified by Debian' 
(`emacs24' on Debian testing).

Francesco