bug#20173: 24.4; Rendering misallocates combining marks on ligatures

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 23 Mar 2015 01:06:26 +0000
> From: Richard Wordingham <richard.wordingham@ntlworld.com>
> 
> When a ligature of two base characters has two combining marks on the
> first component but none on the second, the second combining mark is
> rendered as though it applied to the second component. A good example
> is the Arabic sequence لَّا (lam, shadda, fatha, alef - <U+0644, U+0651,
> U+064E, U+0627), where the shadda is rendered on the lam part of
> lam-alif ligature and the fatha on the alif part.  This problem is not
> restricted to right-to-left scripts; I encountered the problem when
> debugging left-to-right rendering.  Lam-alif is one of the most
> reliably generated ligatures bearing marks on different components.

Is it possible that some rule(s) are missing from the end of
lisp/language/misc-lang.el?  Could you please take a look and see if
something needs to be fixed/added in how we set up the compositions
for Arabic?

Thanks.