update \XC@mcolor in pgf.sty rather than pgfsys-latexml.def

[xworld21]

The change guarantees that \XC@mcolor is updated after color/xcolor are loaded, but also before finishing loading PGF, as opposed to at the end of the preamble. This also improves the usage of \XC@mcolor: in principle, other packages may be adding stuff to it, so it should be extended rather than redefined.

This fixes #2398, and hopefully does not break anything.