GetStringWidth fix

char type is signed :)

Before:
http://i.imgur.com/Qosr4Gn.png
http://i.imgur.com/NlILvPV.png
http://i.imgur.com/17yMuZA.png

After:
http://i.imgur.com/F46jrDB.png
http://i.imgur.com/DUokURq.png
http://i.imgur.com/1YzSVGq.png