bug#57693: 29.0.50; Is there a more reliable version of `char-displayable-p'?

[Eli Zaretskii]

> From: Ihor Radchenko <yantar92@gmail.com>
> Cc: 57693@debbugs.gnu.org
> Date: Sat, 10 Sep 2022 12:48:07 +0800
> 
> At least, individual buffers may have buffer-local
> face-remapping-alist that may completely change the display.

Is this what happened in the case of that user where
char-displayable-p failed to detect that the character cannot be
displayed?

> However, it is not very clear for me how to determine programmatically if
> given character is displayable in buffer.

Make the char-displayable-p test at run time, I guess, i.e. when you
are actually about to insert it.