bug#44950: 28.0.50; 24-bit colors not used in terminal with emacsclient

[Tim Ruffing <crypto@timruffing.de>]

On Sat, 2021-11-13 at 17:10 +0200, Eli Zaretskii wrote:
> > I personally don't care too much. With the Tc fix applied, Emacs
> > works
> > for me, and believe our current code is fine, even though we don't
> > fully replicate tmux's logic respect to Tc. On the other hand, with
> > full support for setrgbf/setrgbb, we'd support pretty much every
> > existing method out there and the diff is not that large.
> 
> I'm not sure I understand: if supporting "Tc" does the job, what does
> the added code gain us?

Well, there are just (too) many standards and without setrgbf/setrgbb,
we don't support all of them. Even with support for Tc, there may be
terminals which can do 24bit but don't have Tc in their terminfo but
only setrgbf/setrgbb.

It's also possible (but very unlikely?) that they support only non-
"standard" escape sequences and thus can have setrgbf/setrgbb with non-
default sequences in terminfo but not Tc because the latter implies the
"standard" escape sequence. 

As I said, I believe the current code does the job. But still, I
wouldn't be surprised if next year some user proves me wrong and
complains here that 24bit doesn't work with their specific rare
terminal/terminfo while it works in other programs (which support
setrgbf/setrgbb).