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 13:10:47 +0000
> Cc: 41626@debbugs.gnu.org
> 
> What I don't understand is why it works in pure RTL paragraphs,
> though. If I yank the Arabic greeting into a new buffer, I can place
> the mark anywhere and nothing happens to the shaping.

Because testing where the face changes while going backwards in a
buffer is "tricky".  See handle_stop_backwards and its callers.  Maybe
there's some bug in the logic there, I don't know.