[TREELIST] Fix brush handle leak in WM_ERASEBKGND.
[THEME] Fix fallback of mcGetThemeSysColorBrush().

It was implemented according to MSDN docs, i.e. it expected the 2nd
parameter in the range TMT_FIRSTCOLOR ... TMT_LASTCOLOR. That is
wrong however. It expects the COLOR_xxxx constants as GetSysColorBrush().

This fixes TREEVIEW painting on Windows 2000.