bug#42521: 28.0.50; Can't include an image in a display string

[Dmitry Gutov <dgutov@yandex.ru>]

(progn
   (erase-buffer)
   (insert " ")
   (setq o (make-overlay (point-min) (point)))
   (overlay-put o 'display
                (concat (propertize " " 'display
                                    (create-image "splash.svg" nil nil 
:max-height
                                                  (default-font-height)))
                        " text after image")))

The script above will end with a blank space instead of an image being 
displayed.

To compare: replace the first 'display symbol with 'after-string, and
the image is displayed.

Why do we need this: if this worked, we would implement icons display in
the default completion popup in company-mode. It's a feature that has 
been requested for a while. See the discussion here:

https://github.com/company-mode/company-mode/pull/996#issuecomment-662743347

Unfortunately, as we seem to have concluded in

   https://debbugs.gnu.org/cgi/bugreport.cgi?bug=18285
   and
   https://debbugs.gnu.org/cgi/bugreport.cgi?bug=20847,

I have to use the `display' overlay property in most of the cases.

P.S. This is not a regression, Emacs 26.3 is the same.

In GNU Emacs 28.0.50 (build 25, x86_64-pc-linux-gnu, GTK+ Version 
3.24.20, cairo version 1.16.0)
  of 2020-07-20 built on potemkin
Repository revision: 4c08c2f45b9bb0265f6d7c3529011dee1b18e843
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Ubuntu 20.04 LTS