Like this site said:
http://www.allegro.cc/forums/thread/592220
It looks like this http://i35.tinypic.com/1zcfleb.png

And a quad with texture (also shown in next screenshot only scaled up from 4x4 used in test). This line does also scale well and remain smooth, as it is loaded with GL_LINEAR filter.
Looks like http://i38.tinypic.com/fjoler.png