bug#41626: 28.0.50; activating mark breaks RTL shaping in etc/HELLO (auto-composition-mode)

[Eli Zaretskii]

> From: Pip Cet <pipcet@gmail.com>
> Date: Sun, 31 May 2020 10:02:40 +0000
> 
> Recipe:
> start emacs -Q etc/HELLO
> move point to character position 358 (ARABIC LETTER SEEN)
> hit C-SPC to activate the mark
> 
> Expected result:
> mark is activated, no change to shaping
> 
> Actual result:
> The character at position 357 is no longer composed with the one at
> position 358. (screenshot with-mark.png) whereas it was composed
> before (screenshot without-mark.png)
> 
> The same thing works fine in a separate buffer in an RTL paragraph.

This is a known problem with how we handle character composition and
faces when they intermix.  At the time, my conclusion was that fixing
this would be next to impossible without changing the way we handle
the "stop position" during iteration through buffer text.

Of course, I could be mistaken.