bug#6718: 23.2; Should align glyphs according to grid in ansi-term

[Jonathan Kleinehellefort]

I came across this when I tried using the font Inconsolata inside
ansi-term.  Inconsolata does not cover a couple of special Unicode
characters, some of which frequently show up in the output of various
terminal applications.

Emacs will then fall back on some other font with completely different
geometry for those, destroying the grid layout of the buffer.

Steps to reproduce:

 1. run "emacs -Q"
 2. M-x term
 4. type "pstree" into the shell
 5. Choose "Inconsolata" as your font

Result:

Characters now have non-uniform width and height. Note that the pretty
tree drawing gets destroyed.

Expected result:

Glyphs should be aligned in a grid.

Using a more comprehensive font (e.g. DejaVu Sans Mono) does not solve
this completely, as you can still get the same problem with e.g. Chinese
characters.



In GNU Emacs 23.2.1 (i486-pc-linux-gnu, GTK+ Version 2.20.0)
 of 2010-05-16 on raven, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.10707000
configured using `configure  '--build' 'i486-linux-gnu' '--build' 
'i486-linux-gnu' '--prefix=/usr' '--sharedstatedir=/var/lib' 
'--libexecdir=/usr/lib' '--localstatedir=/var/lib' '--infodir=/usr/share/info' 
'--mandir=/usr/share/man' '--with-pop=yes' 
'--enable-locallisppath=/etc/emacs23:/etc/emacs:/usr/local/share/emacs/23.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/23.2/site-lisp:/usr/share/emacs/site-lisp:/usr/share/emacs/23.2/leim'
 '--with-x=yes' '--with-x-toolkit=gtk' '--with-toolkit-scroll-bars' 
'build_alias=i486-linux-gnu' 'CFLAGS=-DDEBIAN -g -O2' 'LDFLAGS=-g' 'CPPFLAGS=''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: de_DE.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Term