Add support for RGB color model in ConTeXt

[gucci-on-fleek]

Motivation for this change

According to the manual, the RGB color model should be supported in ConTeXt.

pgf/doc/generic/pgf/pgfmanual-en-tikz-actions.tex

Lines 129 to 134 in 2131043

	\item Specify a new color using |\definecolor|. Only the color models
	|gray|, |rgb|, and |RGB| are supported\footnote{Con\TeX t users
	should be aware that \texttt{\textbackslash definecolor} has a
	different meaning in Con\TeX t. There is a low-level equivalent
	named \texttt{\textbackslash pgfutil@definecolor} which can be
	used instead.}.

However, using \pgfutil@definecolor{some-color}{RGB}{127,127,127} produces an error. A few of the pgfplots libraries use the RGB color model, so they cannot currently be used with ConTeXt.

Modifications

Plain TeX supports the RGB color model by internally converting to rgb.

pgf/tex/generic/pgf/utilities/pgfutil-plain.def

Lines 29 to 37 in 4cba1da

	\def\pgfutil@emu@RGB#1#2,#3,#4\@nil{%
	\begingroup
	\pgfmathdivide@{#2}{255}\let\pgfutil@emu@RGB@r\pgfmathresult
	\pgfmathdivide@{#3}{255}\let\pgfutil@emu@RGB@g\pgfmathresult
	\pgfmathdivide@{#4}{255}\let\pgfutil@emu@RGB@b\pgfmathresult
	\edef\pgf@marshal{\def\expandafter\noexpand\csname\string\color@#1\endcsname{%
	\noexpand\xcolor@{}{}{rgb}{\pgfutil@emu@RGB@r,\pgfutil@emu@RGB@g,\pgfutil@emu@RGB@b}}}%
	\expandafter\endgroup\pgf@marshal
	}

This PR just copies that same code over to pgfutil-context.def.

Checklist

• Code changes are licensed under GPLv2 + LPPLv1.3c
• Documentation changes are licensed under FDLv1.2

[muzimuzhi]

Just for the info, the support for RGB in Plain TeX is addressed in #772.

[hmenke]

Looking around in the code the entire ConTeXt pgfutil could do with some refactoring. For example the gray colormodel just maps to rgb.

[gucci-on-fleek]

I've also changed it so that the gray color model maps to grayscale and not rgb. I can split that out to a separate PR if you'd prefer, but I've bundled it in with this one since it's a 1-line change in the same file.

Manually creating a grayscale color now works, but the named gray colors still map to rgb. This test file:

\usemodule[t][tikz]

\unprotect
\pgfutil@definecolor{test}{gray}{0.5}
\protect

\startTEXpage
    \tikz\draw [test] (0, 0   ) -- (36bp, 0   );
    \tikz\draw [gray] (0, 36bp) -- (36bp, 36bp);
\stopTEXpage

produces this PDF:

10 0 obj
<<
  /Length 11 0 R
>>
stream
1 0 0 1 0.19925 2.643408 cm
q 
0 0 0 RG 
0 0 0 rg 
0.3985 w 
q 
q 
0.5 G 0.5 g % The "test" color maps to grayscale
0.0 0.0 m 
36.00046 0.0 l 
S 
0 0 0 RG 0 0 0 rg 
Q 
Q 
n 
Q 
1 0 0 1 40.293738 -35.999995 cm
q 
0 0 0 RG 
0 0 0 rg 
0.3985 w 
q 
q 
0.900 0.900 0.900 RG 0.900 0.900 0.900 rg % The "gray" color maps to rgb
0.0 36.00046 m 
36.00046 36.00046 l 
S 
0 0 0 RG 0 0 0 rg 
Q 
Q 
n 
Q 
endstream
endobj

[gucci-on-fleek]

I redid the commit to (hopefully) make the "check-commits" CI pass. There weren't any code changes.

[hmenke]

I'd like to have a few ConTeXt tests before merging this so it is essentially also blocked by #1116 (or rather an extension thereof).

[muzimuzhi]

@hmenke Does this mean #1116 will introduce test or test suite that runs with ConTeXt (by setting l3build
variable specialformats.context)?

[gucci-on-fleek]

Would it be possible to get this PR merged? I've rebased it so it should apply cleanly.

I know that the test suite hasn't been finished yet, but I've been applying this patch to my local installation for a two years now without any issues, so this shouldn't cause any regressions. Plus, this change only affects ConTeXt, so the impact of any possible bugs should be minimal.