gdi32: Implement automatic font substitution

[Dan Kegel]

Liu Qishuai wrote:
> This patch appends all fonts on child_list so that if a character is
> not available in the font, other fonts will be automatically used.
> This is how Windows does to get a CJK character when a latin font is specified.
>
> A lot of CJK-related bugs will be fixed after applying this patch.

Excellent!

In case AJ hesitates to apply this patch:
Can you write a conformance test that verifies this behavior?
Also, can you identify a bug in bugzilla this will fix?
- Dan