After 4091aca090 dumping colors with
CSApproxSnapshot was completely broken. That commit stopped changing
&t_Co from reverting changes to colors not made by the colorscheme by
preventing the colorscheme from being re-sourced every time &t_Co
changed. Unfortunately, that stopped the ColorScheme autocmd from
firing, which stopped s:CSApprox() from calculating the colors, which
meant we would dump the same colors regardless of &t_Co, and regardless
of {xterm,konsole,eterm}.
Fix this problem by calling s:CSApprox() explicitly after changing &t_Co
4eb86232c6