gdi32: Use 0xffff as a default character as Vista does

[Reece Dunn]

2008/6/10 Dmitry Timoshkov <dmitry at codeweavers.com>:
> Hello,
>
> this patch fixes the problem reported in the bug 12305. In Wine Tahoma
> character ':' is mapped to glyph index 0x1f, and that seems to be confusing
> Firefox.

> +        TM.tmDefaultChar = pOS2->usDefaultChar ? pOS2->usDefaultChar : 0xffff;

There is a UNICODE_NOCHAR constant for this.

- Reece