0029323: Intersection algorithm produces the curve with oscillation

0029323: Intersection algorithm produces the curve with oscillation

Sometimes the algorithm of purging of extra points in the walking line makes enormous difference in distance between two neighbor segments of the line. This badly impacts the quality of approximation result. This patch balances the difference in distances by forbidding deletion of some points.

1. tests/bugs/modalg_6/bug27615

The reason of the correction is explained in the message ~0072580 (see issue #28557).

2. tests/bugs/modalg_7/bug28892*
   tests/bugs/modalg_7/bug28984

The reason of the correction is explained in the message ~0072583 (see issue #28984).