In the non-CJK case, the margins are half of the width returned by
GdiGetCharDimensions() but only if the client rect's width is above a
certain size.  There is an exception if the client rect is empty if
the font is sufficiently small.

In the CJK case, the margins are independent of the client rect size.
Signed-off-by: Huw Davies <huw@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>