fixed preview size calculation for gtk 2.16 compatibility
fixed preview size calculation for gtk 2.16 compatibility

jmarshallnz/gummi,tonyolag/gummi-vim,jmarshallnz/gummi,tonyolag/gummi-vim,tonyolag/gummi-vim,jmarshallnz/gummi,jmarshallnz/gummi,tonyolag/gummi-vim