Add support for dvisvgm4ht driver in pgfcoreshade

[michal-h21]

Motivation for this change

I think we need to set the shading mode to rgb explicitly with the dvisvgm4ht driver (which is used by default with TeX4ht), otherwise there can be errors if user selects the cmyk option for xcolor:

\documentclass{article}
\usepackage[cmyk]{xcolor}
\usepackage{tikz}
\usetikzlibrary{fadings}
\begin{document}
  hello
\end{document}

Compile using:

$ make4ht -a debug sample.tex

The error:

! Undefined control sequence.
<argument> ...@end@pos }{\pgf@sys@shading@end@rgb
                                                  }{\pgf@sys@shading@end@rgb }
l.17 ...t!100); color(100bp)=(pgftransparent!100)}

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