fixed the full-width issue in drawText

Adding a position to current char in two cases:

1. The previous char is full-width and current char is nether half-width nor a space;
2. The current char is full-width and the previous char is not a space.

Which means:
- If a full-width char followed by another full-width char, each of them takes double positions;
- If a full-width char followed by a space, there will be no need to add a position, vise versa;
- If a full-width char followed by a half-width char, an additional space will be put into them, vise versa;
- All half-width chars will be displayed as before. without extra positions.