user32: Draw standard scroll bars in hovered state only when they were previously painted by DefWinProc().

When an application handles WM_NCPAINT by itself, standard scroll bars are not drawn even if
WS_HSCROLL or WS_VSCROLL is used. In this case, when handling WM_NCMOUSEMOVE, DefWinProc() shouldn't
repaint standard scroll bars to reflect hovered state, otherwise, DefWinProc() draws over the custom
non-client area painted by the application. This is also the behavior on XP.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=51900Signed-off-by: Zhiyi Zhang <zzhang@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>