bug#53729: 29.0.50; Tamil text not shaped in modeline

[Eli Zaretskii]

> Date: Sat, 19 Feb 2022 10:23:32 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: visuweshm@gmail.com, 53729@debbugs.gnu.org
> 
> Please try this much simpler reproducer:
> 
>   emacs -Q
>   C-x b கட்டற்ற மென்பொருள் RET
> 
> At this point you should see a buffer whose name is "கட்டற்ற மென்பொருள்"
> and whose mode line displays the buffer name correctly.
> 
>   M-: (set-buffer-multibyte nil) RET
> 
> Now the buffer's name should display incorrectly on the mode line.
> 
> IOW, the problem happens when the buffer is unibyte, probably because
> we disable text shaping (a.k.a. "character compositions") in that
> case.

I hope I've now fixed this on the master branch, please test.