[pdftex] Color stack, first try

[Hans Hagen]

Hi Heiko,
\
>File "pdftex.ch" implements the pdfTeX primitive
>interface.
>Because color was set by \pdfliteral, this command and its
>node type are also used for the color stack methods:
>
>* \pdfliteral colorset{<color setting>}
>* \pdfliteral colorpush{<color setting>}
>* \pdfliteral colorpop
>* \pdfliteral colorpeek

hm, this will unnecessarry overload \pdfliteral with keywords (i use lots 
of such literals and don't want a slower pdftex), so why not use

   \pdfcolorliteral

Also, there is less change in compatibility problems (i have no plans to 
use color literals in context) or of potential clashes.

Actually, it would be better to let pdftex handle this by means of 
interpreting \specials (there is already such handling and it would conform 
dvi/pdfm); esp since the content is not literal (in th esense that it is 
parsed content).

Hans
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