bug#19496: 25.0.50; term.c, produce_composite_glyh, cmp->pixel_width uninitialized

[Stephen C. Gilardi]

emacs -Q
open an emacs lisp file that contains (lambda ... )
turn on prettify-symbols-mode
the lambda form is converted to (λ ... ), and should be displayed that way, but 
instead it's displayed as
(\
λ
 ...)

This commit is the first on master to show the problem: 
20791069fa34b486c018ba7f27982bdc6ad2a4ea (found by git bisect)

The problem is the calculation of the pixel_width of the composition in the 
buffer. It's set to the pixel_width of the composition in the composition 
table, but that field is not initialized. (There is no mention of pixel_width 
anywhere in composite.c where the table entries are made.) When I check the 
value of cmp->pixel_width, it's somewhere near MAX_SHORT. It should be a small 
integer. Prior to this commit, in my setup, the value is 1 which avoids the 
problem, but isn't a credible pixel width.

An apparently very wide pixel_width for the composition explains the display 
anomaly. The composition appears to be thousands of characters wide and if that 
were true, the anomalous display would have been appropriate.

I don't see an obvious fix. There seems to be some confusing use in the code 
between "width in columns" and "width in pixels". It's possible that the 
pixel_width field in "struct it" is not named precisely for how it’s used--in a 
terminal, the width may actually be used as a number of columns rather than a 
number of pixels.


In GNU Emacs 25.0.50.1 (x86_64-unknown-linux-gnu)
 of 2015-01-03 on localhost
Repository revision: 20791069fa34b486c018ba7f27982bdc6ad2a4ea
System Description:     Ubuntu 14.04.1 LTS

Configured using:
 `configure --without-x'

Configured features:
SOUND GPM DBUS NOTIFY LIBSELINUX GNUTLS LIBXML2 ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Emacs-Lisp

Minor modes in effect:
  diff-auto-refine-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  prettify-symbols-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Prettify-Symbols mode enabled
user-error: Beginning of history; no preceding item

Load-path shadows:
None found.

Features:
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Memory information:
((conses 16 82152 5459)
 (symbols 48 17585 0)
 (miscs 40 36 102)
 (strings 32 12557 4834)
 (string-bytes 1 332483)
 (vectors 16 8665)
 (vector-slots 8 349036 23681)
 (floats 8 65 657)
 (intervals 56 259 25)
 (buffers 976 12)
 (heap 1024 37582 1315))