bug#28824: 26.0.90; display of pbm images broken?

["Roland Winkler" <winkler@gnu.org>]

start emacs -Q
evaluate (arg FILE-OR-DATA of create-image should point to a pbm image)

(defun foo ()
  (interactive)
  (insert-image (create-image "~/foo.pbm")))

M-x foo

This displays the image with emacs 25, but not with emacs 26.0.90:
there is just a small bounding box (smaller than the image) where
the image should be displayed.  It is my understanding that pbm
images do not depend on external libraries.  (Anyway, I built emacs
25 on the same machine without problems.)


In GNU Emacs 26.0.90 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.18.9)
 of 2017-10-13 built on regnitz
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
System Description:	Ubuntu 16.04.3 LTS

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS
NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT
ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 LCMS2

Important settings:
  value of $LC_COLLATE: C
  value of $LANG: en_US.utf8
  value of $XMODIFIERS: 
  locale-coding-system: utf-8-unix

Major mode: Fundamental

Minor modes in effect:
  display-time-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t