Fix major regression in CSApproxSnapshot

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

plugin/CSApprox.vim

@@ -796,6 +796,7 @@ function! s:CSApproxSnapshot(file, overwrite)
         set t_Co=256
       endif
 
+      call s:CSApprox()
       let highlights = s:Highlights()
       call s:FixupGuiInfo(highlights)