No escape-glyph on untranslated Unicode

[Juri Linkov]

Highlighting a character displayed as an escape sequence with octal
codes in `escape-glyph' face is good only when all possible cases are
covered.  Otherwise, it is misleading when some characters with octal
codes are not displayed in `escape-glyph' face.  There is one case
where this doesn't work.

When an untranslated character is displayed in an Unicode buffer,
it is displayed as an octal code, but without `escape-glyph' face.
`C-u C-x =' typed on such a character provides the following information:

      character: \340 (0303, 195, 0xc3, U+00E0)
        charset: [eight-bit-graphic] (8-bit graphic char (0xA0..0xFF))
     code point: [195]
[...]
There are text properties here:
  display              "\\340"
  help-echo            utf-8-help-echo
  untranslated-utf-8   224

This is an interesting approach.  The string with the octal character
code is attached with the `display' text property to the character.
What would be the best way to add the face `escape-glyph' to it?

-- 
Juri Linkov
http://www.jurta.org/emacs/