Is your feature request related to a problem? Please describe.

When saving a canvas as .tex, I often have to later add the begin{document} and usepackages by hand.

Describe the solution you'd like

• SaveAs(".tex", "standalone") adds the statements below in the output file:
• Default behaviour without arguments adds the statements below as comment (prepend %)

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{patterns,plotmarks}
\begin{document}
<----- here the current output
\end{document}

Describe alternatives you've considered

To develop a bash script that modifies the text file using cat / echo. Or a c++ macro.
https://root-forum.cern.ch/t/colored-2d-histograms-in-pdf-output/13822/45

Additional context

#9147