Add basic TikZ compatibility layer

[vlasakm]

This is notably incomplete, because a similar pgf.opm would be needed for completeness. This should however suffice for regular usage.

[vlasakm]

This can be used as a test:

\fontfam[lm]
\load[tikz]

% From PGF manual, section 17.13.2 "Referencing the Current Page Node - Absolute Positioning"
\tikzpicture [remember picture,overlay]
\draw [line width=1mm,opacity=.25] (current page.center) circle (3cm);
\endtikzpicture

\bye

[olsak]

I used your code, thank you.