bug#39133: 28.0.50; Emacs slowdown on special char

[Eli Zaretskii <eliz@gnu.org>]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: 39133@debbugs.gnu.org,  lg.zevlg@gmail.com,
>   mituharu@math.s.chiba-u.ac.jp,  handa@gnu.org
> Date: Fri, 24 Jan 2020 16:41:40 +0100
> 
> >>>>> On Fri, 24 Jan 2020 14:50:10 +0100, Robert Pluim <rpluim@gmail.com> said:
>     Robert> Yes, I worked that out. A simple crash, for once :-)
> 
> >From 667f47abecc13e8a47181f338e727d95e57a6354 Mon Sep 17 00:00:00 2001
> From: Robert Pluim <rpluim@gmail.com>
> Date: Fri, 24 Jan 2020 14:11:44 +0100
> Subject: [PATCH] Don't attempt to cache glyph metrics for FONT_INVALID_CODE
> 
> This was causing massive slowdown in redisplay when eg #xfe0f
> (VARIATION SELECTOR-16) was present, as the cache ended up very large,
> unused, and being recreated on every call to font_fill_lglyph_metrics
> (Bug#39133).
> 
> * src/composite.c (fill_gstring_body): Hoist FONT_OBJECT_P check out
> of loop.  Calculate glyph code and check for FONT_INVALID_CODE before
> calling font_fill_lglyph_metrics.  Pass glyph code to it.
> 
> * src/font.c (font_fill_lglyph_metrics): Add code parameter, move
> glyph code calculation up the call stack into fill_gstring_body.
> 
> * src/font.h: Adjust font_fill_lglyph_metrics prototype.

Thanks.