[geometry] fixed issue (found myself with testcase #buffer_mp1) where two specific circle-like polygons were not overlayed correctly. This was caused by sections not containing all points. We now make the section boxes a little smaller, such that they are not disjoint (10 * epsilon). This margin is a little fuzzy, but that is not harmful, they only influence might be some more comparisons. But that is intentional, we missed now one.

svn path=/trunk/boost/geometry/; revision=83474