bug#33729: 27.0.50; Partial glyphs not rendered for Gujarati with Harfbuzz enabled (renders fine using m17n)

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 24 Dec 2018 04:08:47 +0200
> From: Khaled Hosny <dr.khaled.hosny@gmail.com>
> Cc: rgm@gnu.org, far.nasiri.m@gmail.com, behdad@behdad.org,
> 	33729@debbugs.gnu.org, kaushal.modi@gmail.com
> 
> I think we are almost good now. There is only one serious FIXME left:
> 
>   /* FIXME: guess_segment_properties is BAD BAD BAD.
>    * we need to get these properties with the LGSTRING. */
> #if 1
>   hb_buffer_guess_segment_properties (hb_buffer);
> #else
>   hb_buffer_set_direction (hb_buffer, XXX);
>   hb_buffer_set_script (hb_buffer, XXX);
>   hb_buffer_set_language (hb_buffer, XXX);
> #endif
> 
> We need to know, for a given lgstring we are shaping:
> * Its direction (from applying bidi algorithm). Each lgstring we are
>   shaping must be of a single direction.

Communicating this to ftfont_shape_by_hb will need changes in a couple
of interfaces (the existing shaping engines didn't need this
information).  I will work on this soon.

> * Its script, possibly after applying something like:
>   http://unicode.org/reports/tr24/#Common

Per previous discussions, we decided to use the Harfbuzz built-in
methods for determining the script, since Emacs doesn't have this
information, and adding it will just do the same as Harfbuzz does,
i.e. find the first character whose script is not Common etc., using
the UCD database.  I think it was you who suggested to use the
Harfbuzz built-ins in this case.

> * Its language, is Emacs allows setting text language (my understand is
>   that it doesn’t). Some languages really need this for applying
>   language-specfic features (Urdu digits, Serbian alternate glyphs, etc.).

We don't currently have a language property for chunks of text, we
only have the current global language setting determined from the
locale (and there's a command to change that for Emacs, should the
user want it).  This is not really appropriate for multilingual
buffers, but we will have to use that for now, and hope that in the
future, infrastructure will be added to allow more flexible
determination of the language of each run of text.  (I see that
Harfbuzz already looks a the locale for its default language, but
since Emacs allows user control of this, however unlikely, I think
it's best to use the value Emacs uses.)  I will work on this as well.

Thanks.