Here (this was not translated to English up to now):
I wrote about the OpenCAD's circle rendering problems. I don't know if they already corrected it, or just I wasn't find the solution.
Now I started to work with OpenSCAD and faced this problem once more. I started to search about it. Finally I found out that circle and the cylinder function has a $fn argument. Setting to $fn=100 gives a very nice result.