Fix an Xutf8DrawString issue when using ISO10646 fonts.
Xutf8DrawString doesn't draw string correctly when using iso10646 fonts.
A Simplified Chinese character is used to illustrate this issue:
"䲠" is a Simplified Chinese character.
Since none of core X11 fonts contains "䲠", an ISO10646 font is used to draw it: WenQuanYi Bitmap Song CJK Font.
The following image shows the problem and the correctly result after fixed: