Faster balance coefficient weight computation

[danoan]

Currently a whole disk is intersected for every computation of the balance coefficient.

Contours are a sequence of adjacent pixels, so we can optimize this computation. We need to compute a full intersection of the disk at the very first balance coefficient computation. All the others can be derived from a delta from the previous one. This delta refers to the pixels in the contour of the disk.

This improvement removes the quadratic slow down we observe as the disk size increases.