Fixed antialiasing of auto-brush based brushes

This patch does two changes:

1) Add 1 px fading to every auto brush
2) Implement "Auto" spacing mode. In this mode the spacing is calculated
   using a different formula. Instead of usual sp = k * size, we use
   sp = k * sqrt(size). This formula gives an excellent line quality
   and shows quite nice performance
3) Use qreal's for calculation of the spacing instad of using dab's size.
   Dab's size is rather unstable on sizes around 1.0 - 5.0.

TODO:

1) Port Vc implementation of the auto brush to use a new formula
2) Activate "Auto" spacing mode for all the default presets in Krita
3) Port the other brushes

CCMAIL:kimageshop@kde.org