Fixed a "screwed" lines bug

It turned out that our "bended lines" bug consists from two different bugs.
1) Lack of tablet events (only in openGL mode)
2) A bug in Basic Smoothing algorithm (some whirls
   appeared even in QPainter canvas mode).

This patch fixes the second one. The bug was related to the fact that
the size of the control points for the bezier curves was *not limited*.
It resulted in lines bending, screwing and whirling effect. Now the size
of the control lines is always bounded, so the lines became much smoother.

But the first part of the "bended lines" bug is still not fixed, so openGL
mode will produce bends in lines.

CCMAIL:kimageshop@kde.org


The first one is the lack of tablet events in openGL mode, and the second
one was related to the