fixed the full-width issue in drawText

When use `drawText` to draw a string which contain some full-width characters, the characters will cover each other side by side.

This pull request fixed the issue.

Some full-width characters: 
- 这是中文
- ＋－＊％！＠＃￥％＆
- ａｂｃｄ
- ＡＢＣＤ