RLM and LRM are composed?

[Eli Zaretskii <eliz@gnu.org>]

Evaluate this form:

  (aset standard-display-table ?‎ (vconcat "->"))

and then visit a file with this single line:

    Hebrew ‏(עברית)	שלום

The character being set up in the standard-display-table is RLM,
RIGHT-TO-LEFT MARK.  If you are reading this in a GUI session, chances
are it will be displayed as whitespace.  The same character is before
the left paren after "Hebrew".  However, Emacs does not display "->"
instead of it, as I'd expect.  It thinks it does (try "C-u C-x =" on
that character), but it doesn't.

If I step with a debugger through produce_glyphs (in the TTY case) or
through x_produce_glyphs (in the GUI case), I see that the glyph we
produce for displaying this character is not IT_CHARACTER, but
IT_COMPOSITION.

Questions:

1. Why do we display this character as composition?  It is not
   supposed to be composed with anything, AFAIK.

2. Is it a bug or a feature that composed characters don't go through
   the display table?  If it's a feature, what is its purpose?

TIA